Plasma concentrations of testosterone and luteinizing hormone (LH) were measured by radioimmunoassay during the ovulation cycle of the hen [G. domesticus]. The baseline concentrations of testosterone were 100-250 pg/ml, and the maximum concentration of 904 pg/ml was observed 8 h before ovulation. The maximum pre-ovulatory concentration of LH of 2.0 ng/ml, which was approximately twice the basal concentration, was observed 6 h before ovulation. The possibility that testosterone is the ovarian excitation hormone which initiates the ovulation-inducing surge of LH is discussed.