The effects of three cationic amphiphilic drugs on rat brain cytosolic phosphatidate phosphohydrolase and their mechanisms of action were studied utilizing membrane-bound, emulsified, and emulsified sonicated phosphatidate as substrates. With the membrane-bound substrate, chlorpromazine, desmethylimipramine, and propranolol inhibited the activity in a dose-dependent fashion with an IC50 of 30-50 microM. In the presence of the emulsified substrate, chlorpromazine was a more potent inhibitor than desmethylimipramine or propranolol but 200 microM was needed for 50% inhibition of activity. Addition of heat-inactivated microsomes to the emulsified substrate, to simulate the conditions with the membrane-bound substrate, did not alter this value. Both Mg2+ and Ca2+ stimulated the enzyme activity but only Ca2+ counteracted the effect of chlorpromazine. Kinetic studies indicate that chlorpromazine acts as a noncompetitive inhibitor of the enzyme. Emulsified sonicated phosphatidate was a good substrate at low (less than 10 microM) concentrations. It was a poor substrate at 1 mM, but at this concentration chlorpromazine stimulated the activity instead of inhibiting. This drug altered the integrity of phosphatidate vesicle membranes as visualized by electron microscopy. The different results obtained with the three types of substrate indicate the importance of the configuration of phosphatidate for the expression of enzyme activity and for its susceptibility to the action of cationic amphiphilic drugs.