The stimulatory effect of the dihydropyridine derivative, Bay K 8644, on the isolated pregnant human uterus, and its interactions with the calcium channel blockers, nifedipine, verapamil and diltiazem and with the calmodulin inhibitor trifluoperazine were investigated. In uterine preparations showing spontaneous activity, Bay K 8644 (1 nmol/l – 1 μmol/l) produced an increase in the frequency of contractions without effects on their amplitude. However, strong phasic contractions were induced in quiescent preparations. The stimulatory action of Bay K 8644 proved to be insensitive to calcium withdrawal, but was completely prevented in the presence of 1 mmol/l EGTA. Bay K 8644 shifted the inhibitory concentration-response curve of verapamil and nifedipine to the right, leaving the diltiazem-and trifluoperazine-induced effect virtually unchanged. Schild plot analysis revealed a competitive interaction of Bay K 8644 with nifedipine, while the interaction with verapamil was of the nonlinear type. These data demonstrated that the dihydropyridine derivative Bay K 8644 possesses calcium agonistic properties also in the isolated human uterus. Furthermore, the competitive interaction with nifedipine showed the existence of specific dihydropyridine receptors closely associated with the calcium channel.