The isolated perfused rat brain was used for a comparative study of the effects of chloral hydrate and trichloroethanol on cerebral energy metabolism. After a perfusion period of 30 min the brain levels of the following substrates and metabolites were measured spectrophotometrically: P-creatine, creatine, ATP, ADP, AMP, glycogen, glucose, glucose-6-P, fructose diphosphate, α-glycero-P, dihydroxyacetone-P, pyruvate, lactate, glutamate, α-ketoglutarate and ammonia. Furthermore, the concentration of chloral hydrate and trichloroethanol in the isolated brain and in the perfusion medium was measured colorimetrically. Little more than 10% of chloral hydrate in the isolated brain and in the perfusion medium were reduced to trichloroethanol. In intact animals there were about 70% of chloral hydrate transformed. Chloral hydrate and trichloroethanol caused an accumulation of P-creatine, no change in the lactate/pyruvate ratio, an increase of the glucose concentration and a decrease of glucose-6-P level in the isolated brain. The rise of brain glucose level was more pronounced after trichloroethanol than after chloral hydrate. The effects of chloral hydrate and trichloroethanol on brain glucose and glucose-6-P levels suggest an inhibition of brain hexokinase activity by these drugs.