Experiments were designed to assess the effects of energy and protein intake on metabolic function and reproductive efficiency in single and twin suckling beef cows and heifers. A role was proposed for endogenous opioid peptides in mediating the effects of photoperiod, suckling and nutrition on anterior pituitary hormone release, and tested with respect to nutrition. The first two experiments were designed to quantify the effects of pre- and post-partum energy and protein intake, on milk yield and composition, body tissue mobilisation, the post-partum anoestrous interval, and subsequent conception rates to artificial insemination and embryo transfer in single- and twin-suckling beef cows. Cows very quickly adjusted their metabolism and level of performance to be in line with their current level of energy and protein intake. Ovulation and the resumption of normal oestrous cycles occurred soon thereafter. The third experiment tested the hypothesis that post-partum energy and digestible undegradable protein (DUP) intake can influence hypothalamic opiodergic tone and the ability of the anterior pituitary to secrete luteinizing hormone (LH), prolactin and growth hormone in response to naloxone (an opioid receptor antagonist) challenge. Results indicated that high energy diets allowed cows to overcome the opioid mediated block on LH release and resume oestrous cyclicity earlier than cows on low energy diets. High levels of DUP increased the opioid mediated block on LH release. The final experiment examined the interactive effects of body condition and energy intake on hypothalamic opioidergic tone and pituitary responsiveness, in terms of LH release, when challenged with an exogenous source of gonadotrophin-releasing hormone (GnRH). Naloxone was unable to evoke consistent LH responses in this experiment but the LH response to GnRH was influenced by both body condition and energy intake. In conclusion opioid peptides may provide a means by which certain homeostatic regulators interface with homeorhetic hormones released from the anterior pituitary gland.