Sexual dimorphism in the rat hypothalamic-pituitary-adrenal axis was investigated by determination of plasma corticosterone and immunoreactive (I-) ACTH in males and in females at each stage of the estrous cycle. A serial blood-sampling technique enabled assessment of covariation of the two hormones across the full circadian range of their concentrations within individual animals. Distinct diurnal rhythms in plasma corticosterone were evident in all rats, and the degree and timing of this rhythmicity, determined by cosinor analyses, did not vary with gender or cycle stage. There were, however, marked differences in absolute levels of corticosterone across the estrous cycle, with the average daily concentration (mesor) increasing progressively from a minimum at estrus (129 +/- 11 ng/ml) to a maximum 3 days later at proestrus (246 +/- 14 ng/ml). The mesor corticosterone value in male rats (102 +/- 21 ng/ml) was not different from that in estrous females, but was lower than that in females at all other stages of the cycle. In contrast, no gender- or cycle-related differences were detected in absolute levels of I-ACTH, although distinct diurnal rhythms, synchronous with those for corticosterone, were evident in all groups. Accordingly, a strong and positive within-rat relationship between plasma corticosterone and I-ACTH was observed in all groups, but there was a clear shift in the nature of this relationship across the estrous cycle, such that the slope (i.e. concentration of plasma corticosterone per unit concentration of I-ACTH) was minimal in males and estrous females and maximal in proestrous females. In conclusion, this study shows that the extent of sexual dimorphism in resting plasma corticosterone levels is dependent on estrous cycle stage, being absent at estrus and maximal at proestrus. Moreover, this variation in plasma corticosterone was not accompanied by corresponding changes in plasma I-ACTH, suggestive of cycle-related changes in responsiveness of the adrenal cortex to trophic stimulation.