1. The effects of intracoronary bolus infusion of hypertonic saline solution on left circumflex coronary blood flow were examined in sixteen anaesthetized and artificially ventilated pigs whilst preventing changes in heart rate and arterial blood pressure. 2. In fourteen pigs, bolus infusion of 7.5% hypertonic saline solution (2 ml within 30 s) caused a steady-state increase in coronary blood flow without significantly affecting right atrial or left ventricular pressure and its rate of rise (dP/dtmax). Infusing normal saline solution (0.9%) at the same rate and volume in seven pigs did not have this effect. 3. In five pigs, the magnitude and the duration of the response of increase in coronary blood flow were increased in a graded manner by graded increases in the concentration of the hypertonic saline solution between 2.5, 5 and 7.5%. 4. In nine pigs, the response of increase in coronary blood flow to the bolus infusion of hypertonic saline solution was not affected by the blocking agents atropine, propranolol and phentolamine, but it was completely abolished in the same nine pigs by the subsequent intracoronary administration of N omega-nitro-L-arginine methyl ester (L-NAME) which blocks the synthesis of endothelium-derived relaxing factor (EDRF) and in seven pigs by solely giving L-NAME. 5. These results showed that the intracoronary bolus infusion of hypertonic saline solution in anaesthetized pigs caused a coronary vasodilatation which involved mechanisms dependent on the release of EDRF.