The concentrations of total androgen, determined by radioimmunoassay and expressed as testosterone equivalents, were measured in the peripheral plasma of laying hens. Total androgen attained peak values at from 8 to 2 h before ovulation. Peak values were not observed either on days when the terminal egg of a sequence was laid or when oviposition failed to occur during the experimental period.For 12 ovulatory cycles each observed on a different hen, the total androgen concentrations, in testosterone equivalents, ranged from 125 to 1100 pg/ml. For six birds in which ovulation did not accompany oviposition, the total androgen concentrations ranged from 150 to 900 pg/ml. For six birds, which were not in lay during the experimental period, the concentrations of total androgen ranged from 200 to 800 pg/ml.