An analysis of the effects of some natural prostaglandins and of synthetic analogues with more selective actions points to the existence of several receptors for prostaglandins in the airways. Three effects have been studied: the contraction and relaxation of bronchial smooth muscle in vitro and the cough elicited by inhalation of prostaglandin aerosols by the conscious cat. From the rank orders of potency of a group of prostaglandins and analogues, from tachyphylaxis induced by continued exposure to a single species and from antagonism by indomethacin or mefenamic acid, three prostaglandin receptors have been distinguished: (1) for contraction (chi receptor) and (2) for relaxation (psi receptor) of bronchial smooth muscle, and (3) for cough (omega receptor). The natural prostaglandins tested stimulated to different extents each of these three receptors; but synthetic analogues with selective actions on only one or two receptors were identified. It seems likely that (1) prostaglandins or thromboxanes not yet tested will interact with these receptors, (2) other receptors for these substances may be distinguished in the airways, (3) the receptors so far distinguished may also be found in other sites, and (4) the receptor categories described may be subdivisible.