The distribution of vasoactive intestinal polypeptide-like (VIP) immunoreactivity in neurons and processes within the suprachiasmatic nuclei (SCN) of the rat was investigated with light and electron microscopic immunocytochemical techniques. These studies utilized well characterized antisera directed to synthetic vasoactive intestinal polypeptide. Specificity was established by absorption of the antisera with synthetic vasoactive intestinal polypeptide. Neurons and dendrites exhibiting specific VIP immunoreactivity are concentrated in the ventral half of the nucleus, with the greatest concentration of immunoreactive perikarya occurring in the ventral SCN immediately adjacent to and within the optic chiasm (OC). Thin varicose axons containing VIP immunoreactivity are present throughout the SCN. A large number of immunoreactive axons leave the dorsal aspect of the SCN to reach the periventricular hypothalamic nucleus and continue dorsally to form an extensive plexus along the ventral border of the paraventricular hypothalamic nucleus. Other immunoreactive axons project upon the contralateral SCN via the OC. Ultrastructurally, VIP- containing neurons in the SCN are characterized by a spherical to slightly elongated soma and an invaginated nucleus that fills the majority of the cell body. In the soma, peroxidase reaction product is localized on the outer membrane of all cellular organelles. The reaction product of immunoreactive boutons is related primarily to vesicles, and some of these boutons establish axodendritic synaptic contacts in the SCN. The demonstration of VIP-containing neurons in the SCN provides further evidence that this nucleus is composed of a heterogeneous population of neurons which form distinct subfields within it.