The purpose of the present study was to characterize the binding of the Ca2+ channel agonist (+/-)[3H]Bay K 8644 in a highly enriched cardiac membrane preparation and to examine its interactions with other Ca2+ channel ligands. Scatchard analysis showed that (+/-)[3H]Bay K 8644 displayed some 265-fold less affinity for its receptor than did the Ca2+ channel antagonist (+)[3H]PN200-110. Furthermore, (+/-)[3H]Bay K 8644 binding site density was significantly less than that seen for (+)[3H]PN200-110. Kinetic analysis of (+/-)[3H]Bay K 8644 binding revealed biphasic association and dissociation rates. The Ca2+ channel antagonist MDL 12,330A stimulated both (+)[3H]PN200-110 and (+/-)[3H]Bay K 8644 binding by effects on Kd. In contrast, diltiazem inhibited (+)[3H]PN200-110 binding but had no effect on (+/-)[3H]Bay K 8644 binding. Both MDL 12,330A and diltiazem inhibited Ca2+-dependent contraction in rat aortic rings but the combination of these two drugs was less than additive in this regard. We conclude that the binding profile of (+/-)[3H]Bay K 8644 results from the racemic nature of this ligand and that MDL 12,330A and diltiazem interact at sites distinct from one another on or around the slow voltage-dependent Ca2+ channel.