Effect of short‐term exposure to 5 ppm of nitrogen dioxide (NO2) for 3 consecutive days on endogenous rat heart, serum and liver lactate dehydrogenase (LDH) was studied in male rats maintained on water or on10%ethanol solution as the sole drinking fluid for 10 days preceding and during NO2exposure. Animals were pair‐fed. Measurements of hepatic alcohol and aldehyde dehydrogenase were also made. Exposure to NO2had little effect on body or liver weights from corresponding controls maintained on water. Ethanol intake increased liver weight in controls and in NO2‐treated rats. Endogenous heart LDH was significantly increased by the NO2exposure compared to water drinking controls. Ethanol intake negated this effect on LDH in NO2‐exposed animals. Kinetic studies on NO2‐mediated action on heart LDH indicate higher Vmaxand apparent Kmvalues in NO2‐treated rats than in the corresponding controls, compared to normal values in NO2‐exposed ethanol drinking rats and their respective controls. No changes occurred in serum or in liver LDH or in hepatic ethanol and acetaldehyde metabolizing enzymes studied as a function of the NO2exposure or the ethanol drinking. The results suggest that specific alteration of heart LDH without concomitant changes in serum LDH may reflect an early indication of cardiac injury by NO2exposure.