1. We have devised an isolated innervated preparation of rabbit trachea in which parasympathetic fibres can be stimulated either pre- or postganglionically. 2. Contractions induced by transmural stimulation of the tracheal smooth muscle and by stimulation of the proximal end of the vagus nerve were both inhibited by tetrodotoxin and atropine indicating that acetylcholine release from neural fibres was responsible for each contraction. 3. Only the contraction induced by stimulation of the vagus nerve was significantly inhibited by hexamethonium indicating that this response was mediated by preganglionic stimulation. 4. The contractions induced by both pre- and postganglionic stimulation were potentiated by prostaglandin F2 alpha (PGF2 alpha). The potentiation was short-lasting and could not be repeated in the same preparation. 5. In contrast prostaglandin D2 (PGD2) inhibited each form of stimulation-induced contraction in a dose-related manner. 6. In separate experiments PGF2 alpha also potentiated the contractile response to exogenous acetylcholine in isolated non-innervated tracheal segments. This localizes the action of PGF2 alpha to the smooth muscle. 7. These effects of PGF2 alpha may be important in the inflammation which accompanies airway disease.