Subcutaneous injections of estradiol-17 beta and progesterone (.10 and .25 mg/kg of body weight) for 7 (group I) or 21 (II) days were used. Dexamethasone (.028 mg/kg of body weight per day) or adrenocorticotropin (200 IU per day) was injected into cows in each group on days 18 to 20 (I) or 32 to 34 (II). Additionally, 100 mug of thyrotropin releasing hormone was injected intravenously on days 1, 7, 17 (I) or 1, 7, and 31 (II). Milking was initiated on days 21 (I) or 35 (II). Overall 13 of 14 cows had mean daily yields of milk greater than 5 kg; 12 had 305-day lactations. Yields of milk in cows injected for 21 days were greater on day 1 and increased more rapidly until peak was reached at 10 wk; daily mean production throughout lactation was greater (14.3 versus 10.1 kg) than for cows injected for 7 days. Lactation curves pooled within cow within treatment differed. Concentrations of estradiol, estrone and progesterone increased during steroid injections and were 2- to 3-fold higher on day 21 in II than on day 7 (I or II), but concentrations of prolactin and total glucocorticoids in plasma did not differ during this time. The quantity of prolactin released in response to injection of thyrotropin releasing hormone was greater 10 days after steroid injections than before or during steroid injections. Preinjection concentrations of prolactin were correlated with magnitude of postinjection response to thyrotropin releasing hormone, but response was not correlated with concentrations of steroids in plasma on day of injection.