To study the role of the suprachiasmatic nuclei (SCN) in generating circadian rhythms in female rats, lesions were placed in the SCN or in the medial preoptic (PO) region. Serial blood sampling at 4-hour intervals at 3 and 13 weeks after surgery indicated that complete SCN destruction abolished rhythmic fluctuations in plasma corticosterone levels in individual rats. Partial destruction produced less interference, while medial PO lesions that spared the SCN were without effect. Similar effects were noted on daily changes in body temperature at 10 weeks after surgery; however, some rats showed evidence of dissociation of these two rhythmic functions in that some lesions appeared to affect one and not the other. In ancillary studies, it was found that all lesioned groups showed nocturnal feeding patterns similar to those of the controls and that the diurnal pattern in plasma thyrotropin (TSH) levels was altered by complete destruction of the SCN. These data suggest that the SCN are essential for the circadian rhythms in pituitary-adrenal function and body temperature and that separate pacemakers may be present in these nuclei for these two periodic functions. The SCN may also control rhythmic TSH secretion, but these nuclei and the medial PO region do not appear essential for nocturnal feeding.