Radioimmunoassays were used to estimate luteinizing hormone (LH) and progesterone in samples of blood taken from individual hens at frequent intervals throughout their respective ovulatory cycles. A consistent pattern in the plasma levels of both hormones was observed. Significantly more LH and progesterone was present in plasma 4\p=n-\7h before ovulation than at other times during the cycle. An increase in the level of progesterone either preceded that of LH or the two hormones increased simultaneously. At no time did an increase in the level of LH occur before the rise in progesterone.In those birds which did not ovulate during the experimental period the levels of both hormones remained low. The significance of these findings in relation to the neuroendocrine control of ovulation in the hen is discussed.