The relationships of circulating estradiol, estrone, and progesterone levels to plasma lipid levels and ovulatory frequency were studies in sexually mature hens. New Hampshire X Columbian (NH X C) cross-bred hens, which have a relatively low frequency of ovulation, were compared with pure-bred White Leghorn (WLH) hens with a high frequency of ovulation. A mutant strain of WLH hen, exhibiting the phenotypic characteristic of restricted ovulation (RO) was also tested. It was fund that NH X C hens had a basal circulating estrogen: progesterone ratio 2.7 X greater than WLH hens. Furthermore, WLH-RO hens exhibited 3 X HIGHER BASAL PLASMA ESTROGEN AND 4 X lower progesterone than normal WLH hens, resulting in an estrogen:progesterone ratio which was l2.6X higher. In additional, WLH-RO hens did not exhibit any diurnal variation in circulating progesterone levels, and had smaller primary sex organs and also intermittent hyperlipidemia. It is concluded that the basal circulating estrogen:progesterone ratio in sexually mature hens is a good indicator of ovulatory frequency, and that the magnitude of the ratio is determined primarily by the progesterone component.