The haemodynamic effects of the two calcium antagonists, tiapamil and verapamil, were compared in conscious rats with or without myocardial infarction. In control rats without myocardial infarction, verapamil was about 20 times more potent than tiapamil to reduce arterial blood pressure. However, if equihypotensive doses of tiapamil and verapamil were compared, verapamil produced a 2-fold larger decrease of dp/dt<sub>max+</sub> (p < 0.01). Three weeks after myocardial infarction, the negative inotropic effect of both tiapamil and verapamil were potentiated to the same extent. However, if equihypotensive doses of tiapamil and verapamil were compared, tiapamil was still less negative inotropic than verapamil.