Summary The pattern of response of glycoraldehyde-3-phosphate dehydrogenase (GAPDH) and lactate/pyruvate ratio was studied in arterial and venous coronary blood, prior and after infusion of nicotine, in dogs with normal coronary circulation and dogs with coronary obstruction produced by Ameroid rings; at the same time, some of the hemodynamic characteristics of the cardiac activity were evaluated. In dogs with normal coronary circulation, nicotine infusion provokes in the coronary venous blood an increase in GAPDH and a significant increase in the lactate/pyruvate ratio. In animals with chronic ischemia, the nicotine infusion produces a small and inconstant increase in the GAPDH content of the coronary venous blood and no significant modifications in the lactate/pyruvate ratio. In the ischemic myocardium there is evidence of increase in the hexose monophosphate shunt, with decrease in the activity of the glycolytic enzymes and of the tricarboxylic cycle ( Gudbjarnason, et al, 1967 , Gudbjarnason, et al, 1968 , Gudbjarnason, et al, 1968 ). An observation of considerable interest is that the activation of the hexose monophosphate shunt occurs both in the ischemic zone and in that with normal oxygen supply. According to Gudbjarnason et al. (1967) this is explained by the fact that the energy requirements and, above all, the production of ribose-5-phosphate for the synthesis of the proteins necessary for cardiac hypertrophy, are supplied by this metabolic pathway. Glycernaldehyde-3-phosphate dehydrogenase (GAPDH) activity diminishes least among others enzimatic activities with decrease in the ischemic tissue ( Gudbjarnason, et al, 1968 , Gudbjarnason, et al, 1968 ). The importance of this enzyme in the carbohydrate and fat metabolism of the organism in general, and also for the myocardium, is well known: it plays a fundamental role in the activity of the embden-Mayeroff cycle and, indirectly, activates the cycle of the hexose monophosphate shunt in the ischemic tissue ( Gudbjarnason and Bing, 1971 ; Tschopp et al., 1968 ), allowing the conversion of the triose phosphates to acetyl-CoA, facilitating there by also the synthesis of fatty acids. Following up on a series of research initiated by one of us ( Corsini, et al, 1968 , Corsini, 1970 ), the behaviour of GAPDH, parallel with that of lactate/pyruvate ratio, was studied in the arterial and venous coronary blood, prior to and after infusion of nicotine, in dogs with normal coronary circulation, as well as in dogs with coronary circulation restricted by the application of Ameroid. Nicotine was chosen because of its known effects on the cardiocirculatory system (increase of heart rate, blood pressure, stroke volume and reduction in the coronary flow in presence of coronary insufficiency), and also on account of its action on carbohydrate and fat metabolism by stimulating such hormonal systems as the catecholamines and pitressin ( Goodman and Gilman, 1970 ).