Aims: The effects of ethanol on erythrocyte glucose 6-phosphate dehydrogenase (G6PD) activity were investigated under in vitro and in vivo conditions. Methods: For in vitro studies, glucose 6-phosphate dehydrogenase was purified from human erythrocyte and rats were used for in vivo studies. Enzyme activity was determined spectrophotometrically by the Beutler method. Results: The in vitro study showed that the I-50 value was 17 mM for ethanol. In the case of the in vivo study, a 2 ml/kg dose of ethanol significantly inhibited the G6PD activity. The inhibition rate after ethanol administration was 59%, 40% and 6% at 1, 3 and 6 h after, respectively. Conclusions: The results of this study suggest that ethanol has a significant inhibitory effect on the G6PD activity both in vivo and in vitro.