Acid and alkaline phosphatase activities were measured by biochemical methods in endometrial and luteal homogenates from a group of 79 nulliparous gilts for each day of a 20-day cycle. Acid phosphatase activity in luteal tissue was generally higher than alkaline phosphatase activity throughout most of the estrous cycle. A progressive rise in acid phosphatase activity, starting at day 6, reflected maturation and secretion processes within corpora lutea, whereas a sharp rise at day 14 suggested the onset of luteal degeneration. Alkaline phosphatase activity declined progressively as corpora lutea matured and was lowest on day 10. A significant increase in alkaline phosphatase activity occurred between days 10 and 15. The critical rise in alkaline phosphatase activity was related to peak progesterone synthesis by luteal tissues. Alkaline phosphatase activity diminished greatly after day 15 and coincided with luteal regression. A change in the acid and alkaline phosphatase activity ratio between days 13 and 16 appeared to indicate a critical period in the metabolic life of corpora lutea. Endometrial acid phosphatase activity was low when compared to alkaline phosphatase activity and changed little throughout the estrous cycle. Endometrial alkaline phosphatase activity increased markedly during the active luteal phase and decreased below acid phosphatase activity during the follicular phase. The most pronounced drop in endometrial alkaline phosphatase activity occurred after day 10.