Groups of control and Area Postrema rats were treated with 0.1 mg/kg, i.p., DSIP or with saline, at 06:00, 09:00, 12:00, 15:00, 18:00, 21:00, 24:00, and at 03:00. Colonic temperature, blood pressure, and heart rate were measured 30 min after treatment. DSIP induced a shift in the hyperthermic cycle of control rats, but was unable to modify the noncyclic hypothermia found among Area Postrema-lesioned rats. Furthermore, DSIP caused a decrease in the blood pressure level of control rats, but had no such effect on the already depressed level of blood pressure in the Area Postrema-lesioned rats. Finally, DSIP decreased the heart rate of control rats and significantly antagonized the elevated heart rate observed in the Area Postrema-lesioned rats. The data do not permit us directly to relate the physiological change induced by DSIP to its sleep-promoting effects.