In experiments on frog Rana temporaria L. urinary bladder, we investigated localization of NO-synthase (NOS) in urinary bladder slices and measured NOS activity in the suspension of mucosal epithelial cells. Intensive NADPH-diaphorase staining which is widely used as an indicator of NOS activity was found in mucosal epithelium. Almost all mucosal epithelial cells isolated in Ca2+ -free conditions demonstrated positive NADPH-diaphorase reactivity. Direct measurement of NOS activity in suspension of mucosal cells determined by the rate of conversion of L-arginine to L-citrullin showed that the enzyme activity was reduced in absence of external Ca2+ and was inhibited by L-NAME: non-specific NOS inhibitor, and 1400 W: a highly selective iNOS inhibitor (control: 754 +/- 184; L-NAME, 1 mM 329 +/- 87; 1400 W, 20 mM: 547 +/- 25; Ca2+ -free/EDTA: 490 +/- 184 cpm [3H]-citrullin/10(6) cells per 45 min, p < 0.05, n = 7-8). The data obtained demonstrate that frog urinary bladder mucosa epithelial cells provided antidiuretic hormone-induced increase of osmotic water permeability contain nitric oxide synthase. The presence of inducible (iNOS) as well as constitutive isoform(s) revealed in these cells allows to suggest involvement of NOS in intracellular signaling pathways regulated water transport across the epithelium.