The effect of morphine on rat cortical, hippocampal, thalamic, and hypothalamic RNA and ATP concentration was studied. After a single dose (10 mg/kg, i.p.) there was an increase of RNA in the cortex and thalamus, and a decrease of ATP levels in both structures. When the drug was given chronically (daily injections during 10 days at increasing doses from 10 to 100 mg/kg), only a fall of thalamic RNA and an increase of thalamic ATP were observed relative to saline-treated controls. Rats which received the chronic saline treatment, however, had higher hippocampal thalamic RNA and levels and a lower thalamic ATP concentration than those which received a single saline Injection.