We have previously reported that immobilization stress administered twice daily over a one-week period results in delayed puberty and reduced fertility in C57/B16 mice exposed to the stress prior to or at the onset of puberty. Exposure to the same stress in adult cycling or non-cycling females led to larger litter sizes and a greater incidence of conception. The purpose of the present study was to determine whether the effects of stress on fertility were associated with changes in the amplitude or rhythm of corticosterone concentration in the blood. Prepubertal mice subjected to daily immobilization stress showed a shift in the peak of the daily corticosterone rhythm which persisted for at least a week after the stress periods were terminated. In adult female mice housed near males in order to maintain an estrous cycle, the same stress caused a reversal of the daily corticosterone rhythm with peak levels of corticosterone occurring during the midpoint of the light interval rather than shortly after the onset of darkness. Old females maintained in large groups and showing no evidence of regular estrous cycles had no evidence of a 24-h corticosterone rhythm before stress. During the week after stress had been discontinued these mice developed a daily rhythm. It can be concluded that in mice the presence of adrenal rhythm is associated with fertility and the absence of such a rhythm is correlated with infertility.