The receptors mediating prostanoid-induced contraction of guinea-pig isolated trachea have been characterised in terms of a recently proposed general classification of prostanoid receptors. Results obtained on the trachea were compared with those obtained on guinea-pig fundus, which contains a sub-type of PGE2-sensitive (EP-) receptor termed the EP1-receptor, and guinea-pig lung strip, which contains a thromboxane-sensitive or TP-receptor. The following agonists were studied, PGE2, PGF2 alpha and the thromboxane-like agonists U-46619 and Wy17186. The antagonists studied were SC-19220 which selectively blocks EP1-receptors, and AH19437 which selectively blocks TP-receptors. On guinea-pig fundus the rank order of agonist potency was PGE2 greater than PGF2 alpha greater than Wy-17186 approximately equal to U-46619, and responses to all agonists were antagonised by SC-19220 but not by AH19437. On guinea-pig lung strip the rank order of potency was U-46619 greater than Wy17186 much greater than PGF2 alpha greater than PGE2 and responses to all agonists tested were blocked by AH19437 but not by SC-19220. On the trachea, the rank order was PGE2 = U-46619 greater than Wy17186 = PGF2 alpha. SC-19220 antagonised responses to PGE2 and PGF2 alpha, but not those to U-46619 or Wy17186. Conversely, AH19437 antagonised responses to U-46619 and Wy17186 but not those to PGE2 or PGF2 alpha. It is concluded that prostanoid-induced contractions of guinea-pig trachea can be mediated by both EP1- and TP-receptors.