The effect of ethanol "in vitro" on GTP-binding protein in postmortem human brain cortex membranes was analyzed using by photoaffinity hydrolysis resistant GTP analog (azidoanilido GTP, AAGTP). AAGTP identified stimulatory GTP binding protein [Gs alpha (42 KDa)] and inhibitory GTP binding protein [Gi alpha (40 KDa)] in human brain cortex membranes. Ethanol augmented the amount of AAGTP binding to both Gs alpha and Gi alpha in human temporal and parietal cortex membranes. These results show that ethanol "in vitro" elevates not only Gs functions but also Gi functions in postmortem human brain cortex.