Prenatal protein malnutrition significantly elevates brain levels of serotonin in rats, and these levels remain elevated throughout their lives. This biogenic amine is involved in the regulation of many physiological functions, including the normal sleep/wake cycle. The present study examined the effects of prenatal protein malnutrition on the sleep/wake cycle of freely moving adult rats. Six prenatally protein malnourished (6% casein) and 10 well-nourished (25% casein) male rats (90-120-day-old) were chronically implanted with a standard set of electrodes (to record cortical electroencephalogram, neck muscle electromyogram, electrooculogram, and hippocampal theta wave) to objectively measure states of sleep and wakefulness. Six-hour polygraphic recordings were made between 10.00 and 16.00 h; a time when the rats normally sleep. Prenatally malnourished rats spent 20% more time in slow wave sleep (SWS) compared to the well-nourished rats. The total percentage of time spent in rapid eye movement (REM) sleep was 61% less in prenatally malnourished rats compared to well-nourished control rats. These findings demonstrate the adverse consequences of prenatal protein malnutrition on the quality and quantity of adult sleep in rats. These sleep changes are potentially detrimental to normal social behavior and cognitive functions. Prenatally malnourished rats are an excellent animal model to study the role of endogenous serotonin in the regulation of the normal sleep/wake cycle.