1. The biology and chemistry of the endogenous opioid peptide precursors and the distribution, release and metabolism of the products of proenkephalin are summarised, together with a review of the in vitro and in vivo cardiovascular effects of endogenous opioid peptides. 2. The four major products of proenkephalin, [Met] enkephalin, [Leu] enkephalin, [Met] enkephalyl-arg[6]-phe[7] and [Met] enkephalyl-arg[6]- gly[7] -leu[8] , were studied for their direct effects on isolated rat atria and perfused mesentery in vitro and for their indirect effects on responses of the tissues to exogenous noradrenaline. The effects of the proenkephalin products upon atrial responses to noradrenergic and cholinergic nerve stimulation were also investigated. [Met] Enkephalin, [Leu] enkephalin, [Met] enkephalyl-arg6-phe7 and [Met] enkephalyl-arg[6]-gly[7]-leu[8] (10[-9]-10[-6]M) had no direct effects upon isolated atria or the perfused mesentery and did not produce any modulatory effects upon responses to exogenous noradrenaline or on atrial responses to noradrenergic and cholinergic nerve stimulation. 3. The cardiovascular responses to proenkephalin products were investigated in the urethane-anaesthetised rat. Blood pressure and integrated heart rate were measured using a pressure transducer connected to a carotid artery cannula. Intravenous administration of proenkephalin products (30-300?g/kg) produced a dose-related decrease in mean arterial pressure and heart rate. This was of a similar magnitude for the four opioid peptides. The response was abolished by naloxone (1 mg/kg) demonstrating that it was opioid-receptor mediated, and was qualitatively similar to that produced by the p-opioid receptor selective agonist DAGO. The involvement of u -receptors in the response is further supported by the lack of effect of the ?-receptor selective agonist DPDPE and k-receptor selective agonist U-50, 488H. This militates against an involvement of ?- and k-receptors in the cardiovascular response to proenkephalin products. 4. An "atypical response" to [Met] enkephalyl-arg[6]-phe[7] was exhibited in 25% of preparations studied. This consisted of an initial bradycardia and decrease in mean arterial pressure followed by a tachycardia and increase in mean arterial pressure. The tachycardia and pressor response were mimicked by the dipeptide arg-phe and antagonised by propranolol and phentolamine respectively. Both components of the response were abolished by hexamethonium which suggests that the response may have been due to the stimulation of sympathetic ganglia. The "atypical response" was produced in litter mates and may reflect genetically controlled differential capabilities to enzymatically degrade the heptapeptide and generate the pharmacologically active dipeptide. 5. The cardiovascular responses to proenkephalin products were potentiated following pre-treatment with captopril, the angiotensin converting enzyme inhibitor, or bestatin, the aminopeptidase inhibitor, although to a lesser extent. The effects of captopril on the duration of the responses, especially to the heptapeptide and octapeptide, were most marked indicating that the activity of a captopril-sensitive enzyme may be responsible for the relatively short duration of the observed responses. 6. Pharmacological manipulations, such as the use of atropine and the quaternary opioid antagonist N-methyl levallorphan, and surgical procedures, including bilateral vagotomy and pithing, were employed to examine the mechanisms involved in the cardiovascular response to proenkephalin products. Through these techniques it was demonstrated that the response was vagally dependent, mediated by peripheral opioid receptors and dependent on the central nervous sytem functioning. The cardiovascular response to proenkephalin products therefore arises from the stimulation of peripheral opioid receptors. This is then transmitted to the central nervous system by the vagus nerve and results in the production of a centrally mediated decrease in mean arterial pressure and an atropine-sensitive bradycardia. 6a. Non-opioid effects of N-methyl levallorphan (initial nicotinic agonist activity followed by ganglion blocking effects) were observed at doses only two-fold higher than those required to block cardiovascular responses to Met enkephalin. The narrow selectivity of this compound makes it inappropriate for opioid antagonist studies where nicotinic receptors may be involved. 7. Manipulations of the hypothalamus-pituitary-adrenal axis, through adrenalectomy and suppression of pituitary function by dexamethasone administration, had significant effects upon the cardiovascular responses to proenkephalin products. This indicates that the activity of the hypothalamus-pituitary-adrenal axis and/or changes in the levels of circulating opioids has a considerable influence on the magnitude of the evoked response. 8. The respiratory effects of proenkephalin products and cardiovascular responses in artificially ventilated rats were studied. The results confirmed that the cardiovascular responses to proenkephalin products in the urethane-anaesthetised rat were not secondary to respiratory effects.