It has been demonstrated in unrestrained rabbits that the delta-sleep-inducing peptide (DSIP, 60 micrograms, intraventricularly) produced sleep lasting 55-65 min. At that time the amount of delta-waves on the EEG was 230 % of the initial level. Of the 22 structural analogs of DSIP, the DPhe-containing compound compared favourably with the delta-sleep-inducing peptide as regards the activity. Other analogs were either less active or had no hypnogenic activity at all.