The effect of age and duration of injection of prolactin on the slope of the dose-response curve in the systemic crop sac assay for prolactin in pigeons was studied. In 6-week-old pigeons the slope at 4 days was 2.5 and at 7 days it was 5.4 g of crop sac weight increase per 10-fold dosage change. In 5- to 8-yr-old pigeons the slope at 4 days was 6.5 and at 7 days it was 17.4. The index of precision increased with the level of response of crop gland weight. In old pigeons at 7 days, and at crop gland weights below 5 g, λ was 0.05–0.1 or about 25% of that in young pigeons at 4 days.