Relaxing effects of the nitric oxide donors GEA 3175 (3-aryl-substituted oxatriazole derivative), SIN-1 and sodium nitroprusside (SNP) were compared in the rat bronchial rings in vitro. In epithelium intact rings, after metacholine precontraction ED50 of GEA 3175 and SNP were similar (2 and 3.5 microM respectively) while the maximum effect of the former was bigger (92% and 54% respectively). SIN-1 was less potent (ED50 50 microMx, maximum 55%). In the absence of the epithelium the effect of GEA 3175 was attenuated, while that of SIN-1 or SNP were abolished. In the KCl precontracted rings the effects of all compounds were smaller than after metacholine precontraction. Removal of the epithelium did not alter the effects of GEA 3175 or SIN-1 but clearly increased that of SNP (change of EC50 from 10 to 0.7 microM).