The effects of therapeutic concentrations of antihypertensive drugs on catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities in term placental explants were studied. The enzyme activities were measured in tissue fractions using radioen-zymatic techniques. At 6 h the incubation COMT activity increased significantly following exposure to verapamil and hydralazine, while exposure to alpha methyldopa caused a significant suppression of the enzyme. At 24 h exposure to hydralazine significantly suppressed the COMT activity. By 6 h the MAO activity was significantly suppressed by verapamil. MgSO4 and propranolol had no effect on the activities of COMT and MAO at 6 and 24 h. These results suggest that COMT and MAO activities in the placenta can be modulated by antihypertensive drugs and, therefore, these drugs might affect local catechol metabolism.