IN adult rat, immunostaining for pituitary adenylate cyclase activating polypeptide (PACAP) was demonstrated in the soma and dendrites of Purkinje cells and in nerve fibres around granule cells. PACAP in the Purkinje cells was confirmed by the presence of mRNA. The concentration of PACAP-38 was high after birth and declined to adult levels within a few weeks. At birth PACAP-immunoreactive nerve fibres and a few cells were present in the Purkinje cell layer, but already at postnatal day 7 an adult PACAP immunostaining pattern was found. The findings suggest that PACAP could be a neurotransmitter in the adult cerebellum and provide a morphological correlate for the reportedin vitroanti-apoptotic PACAP effects on cerebellar granule cells from 1-week-old rats.