The influence of serotonergic inputs on the levels of vasoactive intestinal peptide (VIP) mRNA in the rat suprachiasmatic nucleus (SCN) was examined by in situ hybridization combined with morphometrical analysis. Depletion of serotonin by p-chlorophenylalanine methyl ester (300 mg/kg i.p. daily for 3 days) caused a marked decrease in VIP mRNA signal levels in the ventrolateral part of the SCN. This finding suggests that serotonergic input controls VIP production at the mRNA level.