Examination of the levels of plasma corticosterone in laying hens every 3 h for 24 h showed that the individual variation was considerable but that there was a clear daily rhythm. A maximum at the end of the dark period and a minimum at the beginning of the night were found (2.3 and 0.5 ng/ml plasma, respectively). Although the maximum occurred at the end of the dark or at the beginning of the light period, the rise of corticosterone levels clearly began before photostimulation could have had any influence. The influence of egg laying was examined with cannulated birds. The increase in corticosterone content was not unexpected because the birds cannot perform their normal laying behavior in cages and are usually in a state of agitation before an egg is laid. Egg laying has an influence on corticosterone levels in the blood.