Determination of dose/response relationships for E and F PGs in guinea pig trachea and human bronchial muscle in vitro indicates that mixed contractant and relaxant activities are present in different proportions in each PG. Further analysis of these effects, using (a) tachyphylaxis to individual PGs and (b) indomethacin as a PG antagonist, argues that two PG receptors exist on tracheobronchial muscle. Activation of one of these (chi receptor) leads to contraction, and of the other (psi receptor) to relaxation. Individual E and F PGs interact with these two receptors in different proportions. A third potent effect of PGs on airways is to elicit cough. That PGF2 alpha elicits cough but has very little relaxant activity, whereas certain PG analogs and PGE1 elicit cough but possess either no bronchomotor or almost entirely relaxant activity, argues that a third receptor (omega) exists, the activation of which elicits cough.