The herbicide, paraquat (Pq), has been reported to decrease the amount of pulmonary surfactant (dipalmitoyl lecithin) in rat lung. We have found that a beta adrenergic blocking agent, propranolol (Pr), reduces about 50 percent the 48-hr mortality of rats from Pq (25 mg/kg, i.p.). Since Pr modifies phospholipid metabolism, we have therefore studied in vitro the effects of Pq, Pr, and P plus Pr on the incorporation of P32-PO4 by rat lung slices. Pq increased by 40 percent the amount of PO432 incorporated in 30 min into phosphatidyl inositol (PI) and decreased the amount in phosphatidyl glycerol (PG) by 20 percent. Pr decreased the amount of P32 in phosphatidyl choline (PC) by 70 percent and increased the amount in PG by 130 percent, in phosphatidic acid (PA) by 390 percent, in lyso-PA by 400 percent, in lyso-PC by 390 percent, and in cytidine diphosphate (CDP)-diglyceride by 1000 percent. The combination of Pq plus Pr reduced P32 in PG by 10 percent, in phosphatidyl ethanolamine by 50 percent, and in PC by 90 percent. Pq plus Pr increased the amount of P32 in PA by 500 percent, in PI by 160 percent, in lyso-PA by 460 percent, and in CDP-diglyceride by 780 percent. The amount of P32 incorporated into dipalmitoyl lecithin was reduced 57 percent by Pq, 70 percent by Pr, and 94 percent by Pq plus Pr. There were no significant differences in the total ATP content of the groups at the end of the incubation period. The in vitro changes do not explain the protective effects of propranolol in vivo.