DSIP and its analogues, [D-Trp1]-DSIP, [D-Tyr1]-DSIP, and [D-Trp1]-DSIP1-6, were injected ICV (7 nmol/kg) into rats at dark onset, and the sleep-wake activity was recorded during the 12-hr dark period and the subsequent 12-hr light period. The effects were evaluated with respect to baseline records obtained after artificial CSF injections. DSIP did not increase sleep, whereas both [D-Trp1]-DSIP and [D-Tyr1]-DSIP promoted sleep in the first part of the night. [D-Trp1]-DSIP1-6 had a prompt arousing effect. It is suggested that the sleep-promoting analogues act by facilitating slight endogenous sleep tendencies at some time after dark onset, while DSIP is degraded quickly and is therefore not effective. The increase of W after [D-Trp1]-DSIP1-6 may indicate that DSIP contains a fragment with an arousing effect. The results corroborate the notion that the active DSIP molecule has a pseudo-cyclic structure.