Protected nonapeptide--Delta Sleep-Inducing Peptide (DSIP) formula: see text has been synthesized by classical method. The product has been treated with TFA and purified on DEAE-Sephadex-A25 column, pure free nonapeptide obtained and alpha to beta transposition of Asp-residue found to be absent. It has been assayed by electrophoresis at pH 3.8, microcrystallinecellulose TLC and HPLC. The physiological activities of synthetic DSIP are performed on rabbits by using intravenous administration or mesodiencephalic ventricular infusion. Its function of intensifying delta and sigma waves on rabbit's electroencephalogram (EEG) is evident. There is no concomitant increase of delta- and sigma-enhancing effect following mesodiencephalic ventricular infusion of 10 or 20 times higher than 5 microgram/rabbit doses. Results of 6-day consecutive intravenous administration (50 microgram/kg) indicate that there is no obvious sign of adaptation to DSIP. Results suggest that the physiological function of endogenous sleep-inducing peptide is different from that of general sleeping draught.