SUMMARY1. The effects of quinidine on blood flow rate and developed tension were studied in isolated, blood‐perfused papillary muscle preparations of the dog. Drugs were injected into the anterior septal artery.2. Quinidine caused a dose‐related increase in blood flow rate; the mean dose producing a 100% increase in blood flow rate was about 0·3 mg.3. Quinidine in doses of 0·01–0·1 mg produced a positive inotropic response. With 0·3–1 mg of quinidine the positive inotropic response was preceded by a transient negative inotropic response. With 3 mg there was a monophasic negative inotropic response, the developed tension being reduced by about 40% of control.4. Propranolol had no statistically significant effects on the responses of the blood flow rate and developed tension caused by quinidine.5. These results indicate that quinidine has an action on coronary vessels in lower doses than on the myocardium, and that in low doses it has a positive inotropic action rather than the well‐known negative inotropic action exerted with