Previous observations indicate that both interleukin 1 beta (IL-1 beta) and insulin are involved in sleep regulation. IL-1 beta has been reported to stimulate insulin secretion, suggesting that some of the effects of IL-1 beta are mediated by insulin. The purpose of the current experiments was to study the possible role of endogenous insulin in physiological sleep regulation and in the hypnogenic effects of exogenously administered IL-1 beta. Isotonic saline or IL-1 beta (2.5 ng) was intracerebroventricularly injected into normal and streptozocin-diabetic rats at dark onset, and sleep-wake activity and brain temperature were recorded for 24 h. Blood samples were collected every 20 min during the first hour postinjection for plasma insulin measurement. In diabetic rats, rapid-eye-movement (REM) and non-REM sleep were significantly decreased during the second part of the light period on the baseline day. IL-1 beta elicited a biphasic sleep response in both normal and diabetic rats. This initial increase in non-REM sleep during the first 4 h postinjection was followed by significant sleep suppression in both groups. Brain temperature was not affected in either group. Plasma insulin concentration decreased in response to IL-1 beta in normal rats, whereas insulin was below the level of detection in the diabetic rats. These results indicate that, although sleep is disturbed in diabetic rats, pancreatic insulin might not have a decisive role in the regulation of sleep in rats, and it does not mediate the effects of IL-1 beta on sleep-wake activity.