The study investigated in vitro effects of halothane, isoflurane, ketamine, sevoflurane, prilocaine, diazepam, and midazolam on enzymatic activity of human red blood cell glucose-6-phosphate dehydrogenase (G6PD; EC 1.1. 1.49). G6PD was purified from human red blood cells by 2’, 5’-ADP-sepharose 4B affinity gel. Enzymatic activity was spectrophotometrically measured at 340 nm according to the method of Beutler. I50 values were determined from drug activity (%)–drug concentration curves. I50 values were as follows: 0.72 mM for isoflurane, 1.82 mM for sevoflurane, 0.38 mM for diazepam, and 0.0019 mM for midazolam. But halothane, ketamine and prilocaine had no inhibitory effect on the G6PD activity in in vitro.