The effects of continuous intravenous (i.v.) or intraperitoneal (i.p.) infusion of regular insulin on food intake (FI) and body weight (BW) were examined. When rats were infused i.v. with insulin at 0.2 IU/h for 14 days, BW increased until the 10th day and then plateaued, whereas FI was augmented until the end of treatment. The 24-h hyperphagia was mainly due to a diurnal increase in FI--resulting mainly from a large augmentation in meal number, with unchanged meal size. Nocturnal meal number increased and meal size decreased so that nocturnal FI was not affected. At the cessation of infusion, rats became hypophagic and lost weight. Another group of animals, infused with a lower dose (0.1 IU/h), showed that the increases in BW and FI were dose-dependent. Insulin infused (0.2 IU/h) for 14 days via an i.p. catheter increased FI and BW. The 24-h increase in FI was again mainly due to an enhanced diurnal intake, but at the cessation of insulin infusion rats did not lose BW. In rats infused with insulin at 0.1 IU/h for 14 days, BW and FI did not increase. In rats infused via an implanted minipump with insulin at 0.1 IU/h for 7 days, however, BW increased significantly over controls for the first 3 days, without any significant change in FI, the discrepancy between results observed here and other results is discussed, these effects suggest a relative ineffectiveness of the i.p. route compared to i.v.