Summary: Glucocorticoid feedback was manipulated in rats deprived of REM for six days or left undisturbed in their home cages. One half of each group received concurrent dexamethasone (400 /-Lg/kg) while the other half received corn oil. On test day, rats were observed for 5 min, injected with apomorphine (0.8 mg/kg, i.p.), and again observed. Dexamethasone reversed REM-deprivationenhanced stereotypical gnawing, but had no effect on REM-deprivation-reduced rearing, suggesting the involvement of catecholamine depletion by non-specific stress-related factors in the former REM deprivation effect. Key Words: REM sleep deprivation-Apomorphine-Dexamethasone.Various pharmacological studies, using the small pedestal (SP) technique, have reported REM sleep deprivation (REM dep) effects on neurotransmitter systems and drug-induced behaviors (1-12). REM dep is believed to increase serotonin turnover, supposed to increase aggressiveness and impair defensive behaviors in rats injected with marijuana and THe. It is also reported to increase striatal dopamine and 3,4-dihydroxyphenylacetic acid, and to increase or decrease dopamine receptor sensitivity. In particular, REM dep has been reported to alter apomorphine-induced stereotypy (13). The present study determined whether this purported REM dep effect on apomorphine-induced stereotypy was influenced by concurrent dexamethasone administration. METHODS SUbjectsMale Sprague-Dawley rats were housed in standard cages (five/cage), then individually housed and gentled for 3 days prior to REM dep. Rats (n = 32) were run in two separate cohorts because of space limitations, and were 80 or 160 days old at the beginning of REM dep. ApparatusThe small pedestal (diameter 7 cm) was an inverted flowerpot placed in water to within 2 cm of the top. The flowerpots were fixed in white plastic buckets 26 cm diameter X 35