We have studied the rate of surfactant secretion in a newborn rabbit lung slice model. Newborn rabbits were injected with [Me-3H]choline and killed 4 h later. Secretion of phosphatidyl[Me-3H]choline was measured in washed slices from blood-free lungs. There was a developmental increase in the rate of secretion during the period 29 to 31 (full-term) days gestation. This was further increased by labor. Labor had no effect on the rate of incorporation of choline into phosphatidylcholine. Thus, the effect of labor was primarily on secretion rather than synthesis. The effect of labor on surfactant secretion could be mimicked by terbutaline and, as shown previously, by prostaglandin E2. It was abolished by indomethacin and beta-blocking agents. These data show that labor stimulates secretion of surfactant and suggest that this effect is, at least in part, mediated by prostaglandins and beta-adrenergic agents, both of which are known to increase physiologically during labor and birth.