INTRAVENOUS administration of prostaglandin F_2a (PGF_2a) increases bronchial resistance in the cat¹ and air overflow volume in the Konzett–Rössler preparation of guinea-pig lungs in vivo². These effects suggest but do not prove that PGF_2a contracts bronchial muscle. They might be caused by changes in fluid distribution in the lungs. Main³ and Horton and Main⁴ showed that prostaglandins E₁, E₂, E₃, F_1a and F_2a relax cat isolated trachea. Because of the uncertainty about the action of PGF_2a on tracheo-bronchial muscle and because this prostaglandin and PGE₂ have been found in human lungs^5,6, where they may affect the smooth muscle of the bronchial tree, we studied their effects on human isolated bronchial muscle. We also studied the effects of PGE₁ and of mixtures of PGE₁ or PGE₂ with PGF_2a on this smooth muscle preparation.