Rapid eye movement sleep deprivation as a probe in elderly subjects. The effects of a 2-night rapid eye movement (REM) sleep deprivation (RSD) procedure on electroencephalographic sleep and mood were examined in 15 healthy elderly control subjects, 14 elderly patients with endogenous depression, and 15 patients with primary degenerative dementia. Compared with control subjects, both patient groups maintained a higher amount of REM sleep time and REM activity during RSD. Unexpectedly, depressed patients showed little rebound in visually scored or automated REM sleep measures following RSD, and they showed stability of REM activity temporal distribution from baseline to recovery conditions. This contrasted with the rebound in REM sleep activity seen in control subjects, and the more modest increase in demented patients. The RSD was fairly specific, with some impact on delta sleep during the procedure but not during recovery sleep. Mood ratings were unaffected by RSD. These findings demonstrated a greater plasticity of REM sleep regulation in the healthy elderly control subjects and suggested a higher REM "pressure" with a "ceiling effect" in depressed patients. Patients with dementia appeared to have an impaired capacity to respond to the challenge of RSD. 