Atrial and atrioventricular conduction variables were studied at control and at the end of each of six consecutive 45-min diltiazem administration periods in eight closed chest-anesthetized dogs. Diltiazem was given as a bolus (50 micrograms/kg, i.v.) followed by an infusion (0.5 micrograms X kg-1 X min-1); doses were doubled in subsequent periods. The plasma concentrations, measured by gas-liquid chromatography, ranged from 8 to 1400 ng/mL and correlated strongly with the doses (r = 0.92; p less than 0.01). The Wenckebach cycle length, basic conduction time, and functional refractory period of the atrioventricular (AV) node increased proportionally with plasma concentration (respective r = 0.90, 0.89, 0.80; p less than 0.01). The minimum mean plasma concentrations affecting these variables significantly were 37, 83, and 175 ng/mL, respectively. Second or third degree AV blocks developed in all dogs for plasma concentrations between 379 and 1400 ng/mL. In four dogs which were given isoproterenol (0.2 micrograms X kg-1 X min-1), these blocks disappeared within 1 min. Atrial conduction time and functional refractory period were slightly but significantly shortened by diltiazem with mean plasma concentrations of 175 ng/mL and over. His-Purkinje intervals were not significantly changed by diltiazem. Systolic and diastolic arterial pressures were decreased by diltiazem (r = -0.64, r = -0.79; p less than 0.01) starting with a mean plasma concentration of 83 ng/mL. We conclude that AV nodal conduction variables are progressively prolonged with increasing plasma concentrations of diltiazem; plasma concentrations affecting blood pressure and AV nodal variables overlap; and the AV blocks produced by toxic concentrations of diltiazem can be corrected by isoproterenol.