Antigen (ovalbumin)-induced contraction of guinea-pig isolated trachea, which largely resulted from the endogenous release of peptidoleukotrienes, was strongly inhibited by the non-selective phosphodiesterase (PDE) inhibitor theophylline and, more potently, by the selective PDE type IV inhibitors rolipram and Ro 20-1724. It was also strongly inhibited by the PDE type V inhibitor zaprinast, but much less so by the PDE type III inhibitor siguazodan and milrinone. Similar results were obtained in trachea minus epithelium. In contrast to their effects vs. allergic airway smooth muscle contraction, both milrinone and siguazodan potently relaxed leukotriene C4 (LTC4)-induced contraction in isolated trachea from non-sensitized animals. In this assay, rolipram, Ro 20-1724 and zaprinast were less active compared to their effects vs. ovalbumin-induced contraction, whereas theophylline had equivalent potency in the two tests. The relative potencies of rolipram and siguazodan in relaxation of trachea were similar when added prior to or after either LTC4 or histamine. These results suggest that the higher potency of selective PDE type IV & V inhibitors compared with PDE type III inhibitors vs. ovalbumin-induced contraction is due to their greater inhibition of anaphylactic mediator release. The converse is true if we consider their bronchodilator actions, although the superior efficacy of selective PDE type III inhibitors over PDE type IV inhibitors may vary in sensitized vs. non-sensitized animals. The present results are in agreement with a previous study showing that low concentrations of a beta 2-agonist increased the relaxant effect of selective PDE type IV inhibitors in guinea-pig trachea. The present data indicate that prophylactic use of selective PDE type IV inhibitors combined with therapeutic use of low dose inhaled beta-agonist might represent an alternative to the use of antiallergic or steroid therapy in asthma.