Recent evidence suggests that serotonergic mechanisms within the cardiovascular system are activated and may be important in the development of the hyperkinetic circulation and the maintenance of portal hypertension in cirrhotic patients. The in uiuo pressor and positive chronotropic response together with the in uitro contractile responses of aortic rings and portal veins to serotonin were studied in three different rat models of cirrhosis, portal hypertension and jaundice: the portal vein-ligated rat, the carbon tetrachloride-induced cirrhotic rat, the chronic bile duct-ligated cirrhotic rat and the 3-day noncirrhotic, nonportal hypertensive-jaundiced rat. In addition, the activity of the enzyme monoamine oxidase type A was determined in lung homogenates prepared from the four groups of sham and treated animals. In the four different groups of sham-treated or operated pithed rats, serotonin caused a dose-dependent increase in mean arterial blood pressure without any effect on the heart rate. The pressor responses to serotonin in the three models of portal hypertension were significantly attenuated from their respective sham group. In the 3-day noncirrhotic, nonportal hypertensive-jaundiced rats, the pressor response was no different than that seen in the sham-operated rats. No evidence of consistent potentiated or blunted in uitro reactivity to serotonin of arterial rings and portal veins from the four groups of rats was seen. Portal hypertension, cirrhosis and hyperbilirubinemia had no effect on the activity of monoamine oxidase type A. These data demonstrate that portal hypertension is associated with an attenuated pressor response to serotonin. The mechanism of this attenuated response is not linked to altered in uitro reactivity of arterial rings to serotonin or increases in the activity of pulmonary monoamine oxidase. Because the in uitro reactivity of portal veins from portal hypertensive rats was not hypersensitive