Adenosine receptor binding was determined in the brains of rats deprived of rapid eye movement (REM) sleep for 48 and 96 h using [3H]L-phenylisopropyladenosine. Adenosine A1 receptors (Bmax) were significantly increased in the cortex and corpus striatum, and this increase was sleep-specific. Endogenous adenosine was assayed in microwave-fixed brain tissue and no significant changes were found in REM-deprived rats.