Vasoactive intestinal polypeptide (VIP) has been shown to increase the amount of time spent in rapid-eye-movement (REM) sleep both in cats and in rats. In the present study we examined the effect of a newly available competitive VIP-antagonist ([4Cl-D-Phe6-Leu17]-VIP) on sleep-wake patterns in male rats during both the light and the dark phase of 24 h. Continuous intracerebroventricular application of this VIP-antagonist reduced by 44% the amount of time spent in REM sleep during the light period. It is concluded that VIP may play a role in the generation and maintenance of REM sleep.