1. Enzymic and ion-exchange chromatographic analyses were used to measure adenosine triphosphate (ATP), diphosphate (ADP) and monophosphate (AMP) in brain extracts from rats treated with a wide range of centrally acting drugs. Phosphocreatine (PC) was assayed by the acid molybdate method.2. An anaesthetic dose of phenobarbitone caused an increase in brain levels of ATP and PC, and a reduction in ADP and AMP. A convulsant dose of leptazol gave rise to precisely opposite effects. Subanaesthetic (hypnotic) and subconvulsive doses of the two drugs, respectively, produced no alterations in brain nucleotide levels.3. Among the psychotropic drugs, dexamphetamine, LSD and hydroxyzine, at the doses used, caused no changes in brain levels of the adenine nucleotides. Iproniazid and imipramine caused slight increases in the ATP level and ATP / ADP ratio, respectively. Chlorpromazine failed to give rise to any effect in the nucleotides 3 hr after administration, but produced a rise in brain ATP after 6 hr. Reserpine, on the other hand, caused a fall in the ATP/ADP ratio 6 hr after injection.4. These results indicate that some psychotropic drugs can cause small changes in the rat brain ATP/ADP ratio but do not support claims by certain workers that such changes correlate closely with the behavioural effects of these drugs.