Plasma corticosterone in chicks reared under several lighting schedules

1. 1. Plasma corticosterone was determined by radioimmunoassay in 6–7-week-old male broiler type chicks, reared under several carefully controlled lighting regimes. 2. 2. When subjects were grouped by photoperiod of rearing, chicks reared in darkness had significantly lower hormone levels than diurnal controls, or than subjects reared in continuous light. 3. 3. Around-the-clock sampling revealed a diurnal corticosterone rhythm, with high daytime levels and lower night-time levels. This rhythm appeared to be retained in constant light, although phase shifted or free running. 4. 4. Neither analysis by light intensity level nor by lights on/lights off status at the time of blood sampling revealed differences in plasma corticosterone between the experimental groups which could be attributed to these factors.