The effect of Delta Sleep Inducing Peptide (DSIP) and arginine vasotocin (AVT) on motor activity and sleep was investigated in the rat. Motor activity was not significantly reduced during the 24 h following systemic DSIP administration (40-160 nmol/kg i.p.), nor were sleep and the power of the EEG delta-band significantly increased in the 2 hours following injection or infusion of DSIP into the lateral or third ventricle (7-24 nmol). Injection or infusion of AVT into the lateral or third ventricle (10(-15)-10(-19) mol) did not enhance sleep and the power of the EEG delta-band. Nevertheless, the following significant effects indicate an unspecified biological action of the peptides: Dark-time motor activity was reduced 1 and 2 days after DSIP administration (160 nmol/kg), and the activity in the first 4 h after injection (80 nmol/kg) was increased. In addition, the power in the delta-band was reduced after administration of DISP (7 nmol) and AVT (10(-17) mol) into the third ventricle. Thus exceedingly small doses of AVT seem to exert not only endocrine, but also electrophysiological effects. It is concluded that neither DSIP nor AVT qualify as a specific sleep-promoting substance.