To study the pulmonary responses to toxic insult, the biochemical and physiological effects of a known pulmonary toxicant (monocrotaline) were investigated. Monocrotaline is a pyrrolizidine alkaloid obtained from the seeds of Crotalaria spectabilis. When this alkaloid is administered to rats in their drinking water (20 mg/1) for 3 weeks, the lung is damaged, resulting in pulmonary hypertension, inhibition of serotonin transport by the pulmonary endothelium, and right heart hypertrophy. Preceeding the hypertrophy is a doubling of the mass of the lung and right ventricle. The change in mass of the lung preceeds that of the right ventricle. The increases in both organs is characterized by elevated RNA but not DNA. The lung mass increase is not accompanied by changes in collagenous proteins but is accompanied by an 86% increase in total lipids. The right ventricle however, responds to monocrotaline with a 400% increase in collagen protein and no change in lipid content, thereby indicating the lung and right ventricle respond differently to monocrotaline. Time course experiments established that the earliest observable event in monocrotaline induced lung damage is pulmonary edema which develops by day 5 and is resolved by day 10. Monocrotaline metabolites generated by an isolated liver and perfused through an isolated lung do not cause pulmonary edema even at concentrations of monocrotaline metabolites near 1 mM. These metabolites do however, alter the pulmonary endothelial transport of serotonin while other endothelial functions such as norepinephrine transport, angiotensin convertining enzyme and 5'-nucleotidase activities are unchanged. The effect of monocrotaline metabolites on pulmonary endothelial cell transport of serotonin is attenuated when the isolated livers are perfused under conditions which inhibit the formation of metabolites. Therefore, one of the pulmonary effects of monocrotaline that takes weeks to develop in vivo, inhibition of pulmonary endothelial transport of serotonin, can be observed under in vitro conditions. These results also directly demonstrate that the pulmonary damage caused by monocrotaline is a result of hepatic metabolism of monocrotaline to a pneumotoxic form.