The effects of adrenergic agonists on dorsal aortic blood pressure, cardiac output, and coronary blood flow were studied in unrestrained coho salmon, Oncorhynchus kisutch. Resting coronary blood flow was 0.43 ml.min-1.kg body mass-1, which represented 1.1% of cardiac output or approximately 0.5 ml.min-1.g-1 compact ventricular mass calculated on 40% of total ventricle mass. Coronary blood flow was phasic and continuous throughout the cardiac cycle; flow seems to be affected by the ventricular contraction, with a peak flow occurring during diastole and a nadir in early systole. Epinephrine injections into the dorsal aorta resulted in a rapid increase in coronary blood flow in association with a rapid increase in dorsal aortic blood pressure. Subsequently there was also a slower increase in coronary vascular resistance, which could be blocked by phentolamine, indicating an alpha-adrenergic vasoconstriction. Isoprenaline injection produced an increase in coronary blood flow and a large reduction in coronary vascular resistance. The coronary vasodilatation was blocked by propranolol, indicating that it may be partly due to a beta-adrenergic vasodilatation. Preliminary results showed a marked increase in coronary blood flow associated with exposure to environmental hypoxia.