The pharmacokinetic interaction between diltiazem and amiodarone was investigated in dogs. In the presence of amiodarone, diltiazem's AUC values were significantly increased and its total body clearance and volume of distribution at steady-state significantly decreased. This study indicates that cardiac patients on combined diltiazem-amiodarone therapy may indeed be in a high risk situation in regards to the unexpectedly high blood levels of diltiazem induced: with the ultimate introduction of such side-effects as (1) the lowering of blood pressure, (2) A/V block, and (3) sinus node depression. Such cases would require immediate dosage adjustment. Assuming that the data obtained in this study can be extrapolated to humans, a patient's physiological parameters should be monitored at periodic intervals and, more importantly, the patient should report the first sign of any untoward effect.