To determine whether the circadian rhythm in pituitary-adrenal function is abolished after interruption of neural connections of the medial basal hypothalamus (MBH), corticosterone patterns were examined in individual adult female rats following surgical isolation of the MBH (MBHI rats). Non-stress plasma corticosterone levels were assessed fluorometrically in blood samples obtained from a tail vein of each rat at 4-h intervals for 44 h, 7–8 weeks after surgery. The highest corticosterone levels for most intact and sham-operated controls were obtained in samples collected at the period of light-dark transition. Individual MBHI rats demonstrated steroid excursions of normal amplitude that were not linked with time of day; these patterns appeared asynchronous and devoid of a 24-h cycle. The data indicate that major excursions in plasma corticosterone levels persist following surgical interruption of connections of the MBH, but circadian rhythmicity is abolishe