Blood pressure and heart rate effects of prostaglandin E2 and F2 alpha were examined after administering each agent into the left lateral brain ventricle of chloralose-anesthetized cats. Administration of prostaglandin E2 (1 micrograms) resulted in significant, prolonged increases in arterial pressure (25.7 +/- 6.7 mm Hg) and heart rate (19.4 +/- 07.7 beats/min). These responses were mimicked when the same dose of prostaglandin E2 was administered into and restricted to the lateral and third ventricles via cannulation of the cerebral aqueduct, whereas no significant cardiovascular responses occurred with administration into the fourth ventricle. Intravenous injection of prostaglandin E2 resulted in a transient decrease in blood pressure but no change in heart rate. Administration of prostaglandin F2 alpha (1 and 3 micrograms) into the CNS produced no significant cardiovascular responses. The same was true when prostaglandin F2 alpha was administered by the intravenous route. These results indicate that pronounced cardiovascular effects can be produced by administering prostaglandin E2 but not F2 alpha into the CNS, and that the central site of action of prostaglandin E2 is in the forebrain.