Thirteen cycling, nonlactating dairy cows were hormonally treated to induce lactation. Starting 7 days after estrus and continuing for 7 days progesterone and estradiol-17β dissolved in absolute ethanol were administered twice daily. Daily dose level for progesterone and estradiol- 178 was .25 mg and .01 mg/kg body weight, respectively. Intramuscular injections of dexamethasone (.028 mg/kg body weight) were given on days 18, 19 and 20 after initial treatment. Mammary tissue biopsies were taken from 9 cows on days 18, 21 and 23 after initiation of treatment (3 cows each day). Histological analysis was performed (> 30,000 contacts) to determine the average percent tissue area composed of epithelium, stroma, lumen, adipose and duct. Difference in percent area between days 18 thru 23 was as follows: (1) epithelium +11.1%, (2) stroma -25.9%, (3) lumen +13.9%, (4) adipose +1.58 and (5) duct -.74%. Also, an increase of 15 nuclei/alveoli occurred between days 18 thru 23. On day 18 small lipid droplet epithelia predominated but was replaced with mixed lipid droplet epithelia by day 23. Day 18 epithelia had elongated dark staining nuclei. Epithelia on day 23 had spherically shaped nuclei containing several nucleoli and a decreased nuclear/cytoplasmic ratio when compared to day 18 epithelia. Four of the hormonally treated cows were allowed to lactate. Production from these four cows ranged from 33 to 74% of the previous lactation. Production and histological development were variable in cows treated with estrogen, progesterone and dexamethasone. / Master of Science