Delta Sleep-Inducing Peptide (DSIP) was not previously found to possess hypnogenic properties. Its analogue (D-TrpI)DSIP was more stable against aminopeptidases. Its intracerebrovascular administration in doses of 0.7, 7 and 70 micrograms/kg body weight induced a significant increase in slow wave sleep (by 40%; p less than 0.01, U-test) and rectal temperature elevation (by 0.9 degrees C; p less than 0.01) within the following 3 day-light hours, as compared to the control rabbits treated with physiologic saline. Intravenous injections of the DSIP analogue increased slow wave sleep by 25% in a narrow dose range (30-70 micrograms/kg).