The effects of ventromedial hypothalamic (VMH) lesions on the circadian periodicity of blood corticosterone were studied in female rats. The rats were kept on a 12-hour light/12-hour dark illumination regimen and fed ad libitum. Three, 10 and 12 weeks after the VMH lesions, the concentrations of blood corticosterone were measured every 4 h for 48 h in the same unanesthetized rats. Three weeks after the operation, the circadian rhythm in VMH-lesioned rats was disturbed and disappeared. The corticosterone levels at 03:00, 07:00, and 11:00 were significantly higher than those in sham-operated rats. 10 and 12 weeks after the operation, the circadian rhythm, however, was notable (p less than 0.05, p less than 0.05). The elevated mean corticosterone levels over 48 h at 3 weeks after the operation decreased at 10 and 12 weeks. The sham-operated rats showed a significant circadian rhythm at 3, 10 and 12 weeks after the operation (p less than 0.001 in each period) with a peak concentration at 19:00 and through at 07:00. These findings show that the corticosterone circadian rhythms which were disturbed in the dynamic phase after VMH lesions recovered in the static phase of obesity, suggesting that the ventromedial hypothalamus is not an essential biological clock of circadian corticosterone rhythm.