Brain extract was obtained from whole brains of donor rats following four days of paradoxical sleep deprivation (PSD). Intraperitoneal injection of this brain extract in normal (i.e., not pretreated) recipient rats led to a significant increase of paradoxical sleep (PS) which was beginning at the third hour postinjection and still present on the day after treatment. Both PS episode number and PS episode duration were increased by the PSD extract, whereas slow-wave sleep was not influenced. Brain extract of nondeprived control rats, on the other hand, had no significant effects. In the present experiments, an extract volume obtained from two donor brains was transferred to one recipient. Compared to our earlier investigations (one donor brain/one recipient) this higher "dose" of PSD extract exerted a more pronounced and prolonged effect. These results support the idea that some PS-inducing factor(s) accumulating in the brain during PS deprivation may be involved in the regulation of paradoxical sleep.