Labor was initiated within 72 hr in 46 of 54 cattle treated with 20 mg dexamethasone intramuscularly. Parturition was induced in animals injected as early as 246 days postconception although the procedure was more effective later than 255 days of pregnancy. Similarly, 10 or 20mg dexamethasone intramuscularly was effective in initiating labor within 72 hr resulting in parturition in 11 of 23 ewes. Only two of eight ewes injected on days 133-139 lambed within 72 hr postinjection compared to 9 of 15 ewes injected on days 140-142 of pregnancy. Normal gestation length is 148 days for the ewes and 278-288 days for the cattle used in this study. Analysis of peripheral plasma during normal parturition in 10 cows indicated a rise in plasma corticoids on days 4-1 prepartum that was significantly different (� < .001) from levels obtained on days 7-5 prepartum or days 3-7 postpartum. This abrupt elevation in maternal plasma corticoids may result from increased synthesis by the fetal adrenals and provide the signal for termination of the pregnancy.