Since verapamil and contrast media both cause depression in the electrophysiologic function of the heart and both may exert these effects by actions on ionic calcium, the possible interaction of verapamil and intracoronary contrast media on atrioventricular conduction was studied in six dogs using surface electrocardiography and HIS bundle electrogram. The effects of intracoronary injection of standard ionic media (Renografin 76) and low osmolal contrast media (Hexabrix and Hexabrix with calcium [8 mEq/L]) were compared. Each contrast media was assessed in the normal state and at four increasing doses of verapamil (0.125, 0.25, 0.5, and 1.0 mg/kg). The PR and AH intervals were substantially prolonged by intracoronary injection of Renografin 76 in the presence of a 1.0 mg/kg dose of verapamil (172 +/- 41 msec to 724 +/- 48 msec, P less than 0.05 for PR interval, and 182 +/- 41 msec to 734 +/- 51 msec, P less than 0.05 for AH interval), with most animals developing second degree heart block (Mobitz, type I). There was no change in the HV interval. Hexabrix and Hexabrix with calcium did not cause significant changes in PR or AH intervals at similar doses of verapamil. Thus, standard ionic contrast causes severe inhibition of atrioventricular conduction in the presence of verapamil, whereas low osmolal contrast media cause no significant negative dromotropic effects either in the presence or absence of verapamil. The osmolality of contrast media is an important mechanism responsible for the depression in atrioventricular conduction attending intracoronary contrast media in the presence of verapamil.