The direct and metabolic effects involved in coronary vasodilation were considered and the effects of vasodilators on the levels of O, NH3, and CO2 in blood were determinedI.v. injection of heptaminol-HCl (10 mg./kg.) into chloralosed dogs increased the coronary venous blood NH3 level by 65%, and the CO2 level by 9%, and decreased the pO2 value by 35%.Adrenaline (i.v. perfusion of 2 μg./kg./min. for 5 min.) increased the NH3 level by 27% and the COs level by 9%, and decreased the pO2 value by 7%; isoprenaline (2 μg./kg./min. for 5 min.) had a similar effect on the 3 parameters, changing them by +7, +2, and -8%, resp.Aminophylline (i.v. injection of 10 mg./kg.) also increased the total venous CO2 and NH3 levels by 5 and 21%, resp., and decreased the pO2 value by 7%, while papaverine (2 mg./kg.) increased these 3 parameters by 4, 53, and 29%, resp.I.v. injected Persantin (1 mg./kg.) increased the pO2 value by 79% and decreased the CO2 and NH3 levels by 13 and 7%, resp.