Binding of the dihydropyridine calcium antagonist PN200-110 was studied in human myometrial membranes. PN200-110 bound reversibly and with high affinity to membrane fragments. The highest concentration of binding sites was found in the sarcolemma. The benzothiazepine calcium antagonist diltiazem stimulated PN200-110 binding by increasing the amount bound at equilibrium. Kinetic studies detected a fast and slow rate of dissociation in the presence of diltiazem.