Effect of photoperiod on the concentrations of corticosterone and luteinizing hormone in the plasma of the domestic hen.

Ovipositions in hens maintained under schedules of 16h light : 8h darkness (16L : 8D) or 8L: 16D were restricted to a 12h period of the day while in hens maintained on continuous light, ovipositions occurred at times throughout the whole of the 24h day. In hens maintained in continuous light a significant rise in the plasma concentration of corticosterone was observed 11-9h before ovulation/oviposition. In hens in 16L : 8D there was an increase in the concentration of corticosterone in the plasma during the first 1.5h of darkness which, under that schedule, coincides with the onset of the "open period" for the occurrence of the preovulatory release of LH. In hens ovulating the first egg of a sequence the rise in the secretion of corticosterone was only transient and levels fell as preovulatory concentrations of LH rose to a peak. However, in hens undergoing an ovulation late in a sequence or in those not expected to ovulate, the concentration of corticosterone in the plasma continued to rise to attain a maximal concentration 8h later. In hens maintained in 8L : 16D there was a tendency for the concentration of corticosterone in the plasma to increase at the beginning of the open period, 4 h after the onset of darkness, though this rise was much smaller than that observed at the beginning of the open period of hens on 16L : 8D. A further, more pronounced, increase in concentration of corticosterone was observed between 11 and 15h after the onset of darkness. It is suggested that the rise in the concentration of corticosterone at 11-9 h before ovulation in hens in continuous light reflects the beginning of an open period with a free-running circadian periodicity of 26-27 h.