1. Lung resistance was measured by the interrupter method in fifteen patients during infusion of prostaglandin E2 or F2α, for termination of pregnancy.
2. Small but significant reversible elevation in lung resistance occurred in patients receiving both prostaglandins; this was an unexpected finding in the case of prostaglandin E2.
3. The changes described were not harmful in normal subjects, but the possible dangers of intravenous administration of prostaglandins to patients with asthma or other bronchial diseases are discussed.