This study was undertaken in laying hens to investigate the possibility that a diurnal increase in the concentration of plasma corticosterone is directly responsible for timing the preovulatory surge of LH which results in the first egg of a sequence. Provided that the ovary contained a mature follicle, i.m. injection of 0.5 or 2.0 but not 0.1 mg corticosterone/kg stimulated a preovulatory release of LH. The dose of 0.5 mg/kg was less effective than that of 2 mg/kg and induced release of LH in only four out of eight hens. However, it resulted in concentrations of plasma corticosterone which were outside the physiological range. Variations in the concentrations of plasma corticosterone were measured in ten hens on two successive nights for 8.5 h starting at the onset of darkness. The birds were maintained on a lighting regimen of 14 h light/day. The hens were selected so that on the first night there was no preovulatory release of LH of a sequence starting soon after the onset od darkness. No diurnal increase in the concentration of plasma corticosterone was observed during the first 6 h of darkness on either night nor was any increase seen before the preovulatory release of LH. These observations suggest that corticosterone is not directly involved in the timing of the first preovulatory surge of LH of a sequence.