This experiment was designed to determine if entrainment to a light:dark (LD) schedule and the free-running rhythm in constant light are altered by partial lesions of suprachiasmatic nuclei (SCN) cells or SCN output pathways. Twenty-four male golden hamsters were housed under 12L:12D. Hamsters received either lesions (n = 16), sham surgery (n = 4), or no surgery (n = 4), and were placed into individual cages with running wheels under 14L:10D. Each time after 4 weeks, the LD schedule was phase advanced by 6 h, phase delayed by 6 h, and then the animals were exposed to constant dim light. At the end of the experiment, brain sections were processed for peptide histidine isoleucine (PHI) and gastrin releasing peptide (GRP) immunohistochemistry. Alternate sections were stained for cells and fibers. Behavioral results indicate that (a) very few SCN cells and SCN efferent fibers, as labeled by PHI and GRP immunohistochemistry, are necessary for the expression of circadian rhythmicity in wheel running, and (b) damage to pathways rostral to the SCN may be more critical for entrainment and rhythmicity than damage to caudal pathways. Targets of PHI- and GRP-immunoreactive SCN efferent fibers were also identified.