The effect of constant light and constant dark on the circadian rhythm of the concentrations of hypothalamic corticotropin-releasing-factor-like immunoreactivity (CRF-LI), plasma ACTH, and corticosterone was investigated. Groups of rats were maintained under normal light-dark, constant light, or constant dark conditions for 10 days. Rats were then killed over a 24-hour time period and hypothalamic CRF-LI, plasma ACTH, and corticosterone concentrations were determined by radioimmunoassay. Under normal light-dark conditions, troughs in hypothalamic CRF-LI concentrations coincided with peaks in plasma ACTH and corticosterone concentrations. In rats housed under constant dark conditions for 10 days, higher hypothalamic CRF-LI concentrations were detected at 20.00, 24.00 and 04.00 h than at 08.00, 12.00 and 16.00 h. These relatively high hypothalamic CRF-LI concentrations coincided with relatively low plasma ACTH concentrations. The amplitude of plasma ACTH concentrations was markedly attenuated compared to levels of rats housed under normal light-dark conditions. The rats exposed to constant dark continued to demonstrate higher plasma corticosterone concentrations post meridiem than ante meridiem. The peak in plasma corticosterone coincided with the peak in plasma ACTH concentrations; however, the amplitude was normal. In rats maintained in constant light for 10 days, a decrease in hypothalamic CRF-LI concentrations at 20.00 h coincided with a peak in plasma ACTH. The peak in plasma ACTH concentrations was not associated with a peak in plasma corticosterone concentrations. The rhythm of plasma corticosterone concentrations was dramatically attenuated and phase-shifted. Together, these findings indicate that alterations of normal light-dark conditions result in changes in the circadian variation in hypothalamic CRF-LI, plasma ACTH, and corticosterone concentrations. Changes in the circadian rhythm of plasma ACTH concentrations were related to changes in the rhythm of hypothalamic CRF-LI, but an apparent dissociation between the pituitary and adrenal rhythms was observed.