The possibility that REM sleep deprivation induces increased sensitivity of rats to the effects of apomorphine was investigated. Rats were deprived of REM sleep by placing them on 6.0 cm platforms for 3 days. Stress control groups consisting of rats placed on 14 cm platforms (control rats) and groups of nonmanipulated animals (normal rats) were used for comparison purposes. Several doses of apomorphine were administered to these 3 groups of animals, and aggressive behavior, stereotyped behavior, verticalization and body temperature were recorded. All measures indicated that REM-deprived rats were more responsive to apomorphine than the normal and control animals, with the exception of verticalization effect, in which both REM-deprived and control animals reacted similarly. These results suggest that REM deprivation induced supersensitivity in brain dopamine receptors. Other possible interpretations are also discussed.