The concentration of progesterone in milk fat was measured to ascertain whether knowledge of corpus luteum function could be used to increase reproductive efficiency. As they calved, 47 cows were assigned to either a milk-sampled group (24) or an unsampled control group (23). Progesterone concentrations indicative of a functional corpus luteum were first present at an average of 29.4 days postpartum. The average postpartum interval to first estrus was 49 days for cows sampled and 7.19 days for control cows. The postpartum interval to conception was 84.8 days for sampled cows and 113.7 days for control cows. Twenty of 63 (31.7%) ovulations were not associated with behavioral estrus. Cystic ovarian disease with periods of anestrus were observed in five cows. We conclude that failure to detect estrus was a more limiting factor in reproductive efficiency than was anestrus. To determine the reproductive status after breeding, we measured progesterone concentrations on days 19 through 23 postinsemination. Progesterone was higher for pregnant than for nonpregnant cows on all days. However, only on days 20 and 21 was there complete separation of the progesterone ranges in pregnant and nonpregnant cows.