In 156 older adults, day-to-day variations in cortisol diurnal rhythms were predicted from both prior-day and same-day experiences, to examine the temporal ordering of experience-cortisol associations in naturalistic environments. Diary reports of daily psychosocial, emotional, and physical states were completed at bedtime on each of three consecutive days. Salivary cortisol levels were measured at wakeup, 30 min after awakening, and at bedtime each day. Multilevel growth curve modeling was used to estimate diurnal cortisol profiles for each person each day. The parameters defining those profiles (wakeup level, diurnal slope, and cortisol awakening response) were predicted simultaneously from day-before and same-day experiences. Prior-day feelings of loneliness, sadness, threat, and lack of control were associated with a higher cortisol awakening response the next day, but morning awakening responses did not predict experiences of these states later the same day. Same-day, but not prior-day, feelings of tension and anger were associated with flatter diurnal cortisol rhythms, primarily because of their association with higher sameday evening cortisol levels. Although wakeup cortisol levels were not predicted by prior-day levels of fatigue and physical symptoms, low wakeup cortisol predicted higher levels of fatigue and physical symptoms later that day. Results are consistent with a dynamic and transactional function of cortisol as both a transducer of psychosocial and emotional experience into physiological activation and an influence on feelings of energy and physical well-being.loneliness ͉ psychological stress A convincing nonhuman animal literature documents the effects of stress exposure on the physiology and neurobiology of the hypothalamic-pituitary-adrenal (HPA) axis and shows that changes in glucocorticoid levels affect a wide range of related physiological processes and health outcomes (1-3). As a result, considerable attention is being given to the possibility that emotional and physical health disorders in humans may emerge because of extreme or chronic stress exposure and frequent or prolonged HPA axis activation (4, 5).Practical and ethical limitations, however, do not allow human researchers to conduct research with the same degree of causal rigor as in the animal literature; it is obviously not feasible to experimentally impose extreme or chronic stress. Consequently, most research on the associations between stress, HPA axis activity, and physical and emotional well-being in humans has relied on correlational data. Much of this research is also cross-sectional, showing point-in-time rather than longitudinal associations (5, 6).Understanding the role of the HPA axis in human disease processes will ultimately require the longitudinal examination of changes in stress exposure, HPA axis functioning, and disease over the course of months and years. In the present study, however, we show that even examining the day-to-day dynamics of experiencecortisol associations can reveal important information abou...