The levels of LH, as determined by radioimmunoassay, in pituitaries and sera of ewes during the estrous cycle and anestrus are described. The mean concentration of LH (μg/g ±SE) in pituitaries was 112 ±15 on day 1 of the estrous cycle and increased steadily to 374 ±47 on day 9. Concentrations in the pituitary fluctuated from 613 ±67 to 537 ±32 on days 11–15 and increased to 998 ±91 during the proestrus. The concentration of LH in serum between the ovulatory peaks was <2 ng/ml during the estrous cycle. The mean concentration of LH in pituitaries in anestrus from May to August varied from 431 ±27 to 300 ±42. The differences among months were not significant. The concentration of LH in serum (ng/ml ±SE) was .25 ±.059 in May, too low to be detected in June and July, and was 1.20 ±0.397 in August. During the estrous cycle the concentration of LH in the pituitary increased from day 1 to proestrus, while the level in serum remained constantly low. The concentration of LH in the pituitary during anestrus does not...