THE rat suprachiasmatic (SCN) contains a dense plexus of low-affinity p75 neurotrophin receptor (p75NTR) immunoreactivity. In some SCN neurons, p75NTR is co-localized with vasoactive intestinal peptide (VIP). The present study examines the effect of third ventricle administration of 192 IgG-saporin immunotoxin on p75NTR and VIP immunoreactivity in the rat SCN. The 192 IgG-saporin immunotoxin abolished p75NTR immunoreactivity in the SCN. VIP immunoreactivity in the SCN of saporin-lesioned animals was not significantly different from that of control animals. Immunolesions of the p75NTR-ir cell population in the SCN may prove useful in clarifying the role of p75NTR in circadian timing.