The effects of the POMC-derived peptide CLIP [corticotropin-like intermediate lobe peptide; ACTH(18-39)] and its shorter fragments ACTH(25-39), ACTH(18-24), and ACTH(20-24) on sleep were investigated in rats housed under normal 12-h light/12-h dark conditions (0600 light on). CLIP(10 ng) or equimolar doses of CLIP fragments, respectively, were injected intracerebroventricularly immediately before the 8-h recording period (0800-1600). It was found that paradoxical sleep (PS) was increased by CLIP (+20%) as well as by the N-terminal CLIP fragment ACTH(18-24) (+18%) and by the pentapeptide ACTH(20-24) (+25%), whereas the C-terminal fragment ACTH(25-39) was ineffective. Slow-wave sleep (SWS) was not influenced. These results clearly demonstrate that CLIP and its N-terminal fragments have selective PS-enhancing effects. CLIP and/or CLIP partial sequences are possible candidates for endogenous PS-inducing peptides involved in the physiological regulation of paradoxical sleep.