The effects of bronchodilators and smooth muscle relaxants on mechanical responses and lung cyclic nucleotide levels in the isolated hemilung of Rana catesbeiana demonstrate striking differences in intensity and time course of drug action in an unstimulated preparation of airway smooth muscle. Isoproterenol, nitroprusside and nitroglycerin elicit a fast onset relaxation (minutes) with ceiling effects at 20, 22 and 43%, respectively, of maximal relaxation. Theophylline, dibutyryl cyclic AMP and papaverine produce maximal or near maximal relaxation, but require 8 to 32 hr for peak effect. Papaverine-induced relaxation is accompanied by a slow increase in lung cyclic AMP and cyclic GMP and is markedly accelerated by isoproterenol. Theophylline (10(-3) M) produces no change in cyclic nucleotide levels and its relaxant effect is not accelerated by isoproterenol. The hierarchy of relaxant responses suggests drug action at discrete loci in a highly compartmentalized effector chain, with cyclic AMP-dependent mechanisms separable into at least two components. The first is activated by isoproterenol and elicits a rapid, limited response, presumably reflecting an increase in cyclic AMP in a relatively restricted pool. The second is activated by papaverine and elicits a very slow, but complete relaxation, presumably reflecting a more pervasive or diffuse accumulation of cyclic AMP secondary to phosphodiesterase inhibition. The major portion of theophylline-induced relaxation in this preparation appears to be independent of changes in cyclic nucleotide metabolism.