The immediate effects of one week of Delta Sleep-Inducing Peptide (DSIP) administration (6 X 30 nmol/kg b.w., i.v.) and late effects after a follow-up period of one week were investigated in a sleep laboratory study. The population of 18 chronic psychophysiological insomniacs was subdivided into two groups of middle-aged (29–59) and older (60–83) insomniacs. In the middle-aged insomniacs, improvement of sleep to normal values occurred by the end of DSIP administration and was maintained during the follow-up week. In the elderly insomniacs, who showed an initial sleep disturbance double as severe as the middle-aged group, the immediate effect was larger, but full normalization of sleep was only obtained by the end of the follow-up period. Although there was an interaction of sleep pathology and age, a regression analysis yielded clear evidence that the effects of DSIP were correlated to the degree of sleep disturbance. The whole sample of insomniacs showed normal sleep patterns at the end of the investigation.