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Blood samples were collected on several occasions from Sprague–Dawley–derived rats by means of cardiac puncture at 8 h intervals over a 24 h period. Rats were sampled four times before ovariectomy, after ovariectomy and after replacement therapy with oestradiol benzoate. It was discovered that each rat tended to maintain a distinct pattern of serum corticosterone. While some rats developed hormonal rhythms, other rats showed no evidence of such rhythms as indicated by patterns of serum corticosterone concentration. The majority of rats that did not show daily corticosterone rhythms were found to have irregular ovulatory cycles as indicated by vaginal smears. Each characteristic hormonal pattern remained after ovariectomy and subsequent oestrogen replacement although resting levels of corticosterone were lower after ovariectomy and higher after oestrogen treatment than before the operation. It is concluded that the presence of an adrenal rhythm is usually associated with regular vaginal smear cycles.