Following deprivation of paradoxical sleep, stress, measured in terms of Selye's classical indices as adrenal hypertrophy, thymus atrophy, body weight loss and stomach ulceration, was determined in Wistar rats. Deprivation was induced using three different methods: the classical platform, multiple platform, and pendulum techniques. Four control groups were run. No differences between platform and the pendulum techniques were detected, suggesting that the large behavioural differences commonly found following pendulum and platform deprivation are not caused by differences in stress. Multiple platform deprivation produced less stress than the classical platform technique, a finding presumably due to a reduction of movement restriction in the latter group. In a second experiment rats were food deprived both with or without platform or pendulum techniques. Again no differences between the platform and pendulum groups were detected, again confirming that stress is not a determinant in the platform-pendulum contrast. The results of both experiments strongly suggest that, contrary to intuition, deprivation of paradoxical sleep (by any of the three techniques) induces only mild stress. This is in contrast to food deprivation, which is a greater source of stress. Despite distinct changes in stress measures obtained, in neither experiment could any sign of stomach ulceration be detected. This seems to imply that rats of the Wistar strain are less sensitive to ulceration than strains used by other researchers.