In 7 rats 45-86% of the hypothalamic paraventricular tissue was destroyed by electrolytic coagulations. During the first 6 post-operative days the mean daily amount of paradoxical sleep was significantly decreased, and the circadian sleep-walking cycles of paradoxical and slow-wave sleep were abolished. The slow-wave sleep rhythm then became normal, but in paradoxical sleep the acrophase was shifted, and in one animal the rhythm was completely inversed. Thus the hypothalamic paraventricular nucleus seems to play a modulating role in the production of paradoxical sleep and in the generation of its rhythm.