SUMMARY1. The effects on the coronary circulation of intravenous injections of nifedipine were compared with those of glyceryl trinitrate in conscious dogs using chronically implanted Doppler flow probes.2. Each drug causes a fall in arterial pressure and a rise in heart rate, circumflex flow and circumflex conductance.3. Dose‐response curves for peak changes in these variables are quantitatively similar for each drug (six dogs); threshold effects occur with about 0.25 μg/kg, and maximal effects with about 16 μg/kg. However, the change in circumflex conductance induced by 8 μg/kg of nifedipine last on the average of 3.5 min, whereas the effects of circumflex conductance induced by the same dose of glyceryl trinitrate last 1 min.4. The effects of 8 μg/kg of each drug were investigated in 5 dogs treated with propranolol, and propranolol plus atropine. The findings suggest that the rise in heart rate elicited by both drugs is mediated entirely through autonomic mechanisms, whereas 80% of the increase in conductance elicited by both drugs is due to their direct vasodilator action and 20% is attributable to autonomic mechanisms.5. The administration of nifedipine into the buccal cavity in doses suggested for clinical use rapidly induces a fall in arterial pressure and a rise in heart rate, coronary flow, cardiac output and cardiac work; these effects may persist for longer than 1 h.6. It is concluded that nifedipine is a potent, long acting coronary and systemic vasodilator in the unanaesthetized dog. It appears to have haemodynamic effects which resemble those of glyceryl trinitrate in some respects but not in o