Treatment of mice aged 23–25 days with chorionic gonadotrophin induced large amounts of an ovarian alkaline phosphatase activity (phosphatase Ib) kinetically distinct from that of untreated ovaries (phosphatase I). The activities of alkaline phosphatase I and Ib varied with age in untreated mice. Phosphatase Ib appeared when serum luteinizing hormone concentrations increased (days 4–10 and days 35–45), and disappeared when concentrations were low (days 11–35). Injection of human chorionic gonadotrophin induced progressively larger amounts of phosphatase Ib activity between day 19 and day 29. However, gonadotrophin treatment failed to induce this activity on days 10–18 and 30–35. Nevertheless, during the latter period, human chorionic gonadotrophin induced especially large increases in uterine weight. Treatment at different ages with sheep luteinizing hormone plus human pituitary follicle-stimulating hormone induced a pattern of response identical with that induced by human chorionic gonadotrophin, although sheep luteinizing hormone alone was ineffective before 35 days. In contrast, human luteinizing hormone induced a response in the absence of exogenous follicle-stimulating hormone.

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