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Plasma corticosterone (fluorometric assay), pituitary ACTH (bioassay using isolated adrenal cells) and hypothalamic corticotrophin releasing factor (CRF) (bioassay using isolated pituitary cells) were measured singly in groups of six female rats which were killed at 11.00, 15.00, 19.00, 21.00, 23.00, 01.00, 03.00, 05.00, 07.00 and 11.00 h, after 5 weeks of adaptation to a photoperiod of 12 h light:12 h darkness. Locomotor activity was recorded continuously, using actographic cages, and the waking/sleep pattern was recorded by electroencephalography from chronically implanted control rats during the first hours of the light span.

The three hormones measured fluctuated with a 24 h rhythmicity, with extreme values ranging between 4·12 ± 1·42 and 31·78 ± 194 (S.E.M.) µg/100 ml for corticosterone, 4486 ± 269 and 16629 ± 882µu./mg pituitary for ACTH, and 439 ± 20 and 1270 ± 39µu. ACTH production/hypothalamus/105 pituitary cells.

The onset of the ascending phase of the rhythm started during the first 2 h of light for CRF, 2 h later for ACTH, and again 2 h later for corticosterone. Similarly, the estimated acrophase of the rhythms occurred respectively, 9·4 (CRF), 10·3 (ACTH) and 14·4 h (corticosterone) after onset of light. These phase relationships point to a central origin of the adrenal rhythm.

The diurnal activation of CRF at the very beginning of the light phase was concomitant with an almost immediate reduction of the locomotor activity and onset of sleep. These correlations favour the hypothesis of a common temporal control of both the adrenal and the sleep/waking rhythms.