The adult suprachiasmatic nuclei (SCN) express neural cell adhesion molecule (NCAM) that carries polysialic acid (PSA), a carbohydrate polymer which controls plastic cell-cell interactions in neural tissues. Expression of the three major isoforms of NCAM [180 (principal PSA carrier), 140, and 120 kDa) varies with developmental and physiological state. The requirements for NCAM and PSA in circadian timekeeping function were assessed among three transgenic mutant strains with differing expressions of PSA and NCAM. These included: NCAM(tm3Ciw) mice lacking only NCAM 180; NCAM(tm1Cwr) mice lacking NCAM 180, 140, and PSA; and NCAM(tm1Cgn) mice lacking all NCAM isoforms and PSA. Locomotor activity was stably entrained to a 12-h light/dark cycle (LD) in the NCAM(tm3Ciw) and NCAM(tm1Cwr) mutants, but not in NCAM(tm1Cgn) mutants, where the daily onset and offset of activity were irregular or lost. Under constant darkness (DD), the free-running rhythmicity of the NCAM(tm3Ciw) mutants expressing PSA was similar to wild-type controls, but was markedly disrupted in the majority of the NCAM(tm1Cwr) and NCAM(tm1Cgn) mutants lacking PSA. This analysis indicates that PSA and its NCAM carrier are necessary for stable free-running circadian rhythmicity under DD, and that NCAM, but not PSA, is needed for synchronization to LD. Notably, when NCAM 180 is deleted, NCAM 140 compensates as the PSA carrier, and may assume other SCN-related functions of the NCAM 180 isoform.