Vasoactive intestinal peptide (VIP) is a neuromodulator and secretagogue for neuronal survival factors. Moreover, in the suprachiasmatic nucleus (SCN) of mature animals, where the circadian clock resides, day-night cycles are paralleled by rhythmical alteration in the levels of the mRNA encoding VIP. We now show, for the first time, that diurnal oscillation in VIP mRNA occurred in animals reared in the dark and also preceded eye opening. Thus, by Northern blot hybridization elevated levels of VIP mRNA were measured at night time as compared to day time. Furthermore, diurnal oscillation in VIP mRNA were more pronounced in rats reared in the dark. These observations suggested an endogenous as well as exogenous (environmental) control of VIP gene expression in the SCN and implicated VIP as an integral part of the circadian clock.