Testosterone, estradiol and progesterone were measured in peripheral plasma during the estrous cycle of 6 heifers. Estradiol and progesterone results confirmed earlier reports. Concentration of testosterone on the day of estrus was 40 .+-. 3 pg/ml (.+-. SE of the mean), and 2 peaks were detected during the cycle, one 7 days before estrus (1809 .+-. 603 pg/ml) and the other (78 .+-. 7 pg/ml) on the day before the onset of estrus. The concentration of progesterone declined in most cases 1 day after the maximum concentration of testosterone. .beta.-Methasone treatment in 5 heifers extended luteal function by an average of 10 days: plasma androstenedione and estradiol concentrations were unaltered; cortisol values were depressed for at least 16 days after treatment; testosterone concentrations were lowered by 13 .+-. 2.4% during treatment, and except in 1 heifer the peak on day 7 was abolished.