There are many reports on acute renal failure (ARF) after ingestion of grass carp bile (CB; Ctenopharyngodon idellus). Renal dipeptidase (RDPase; EC 184.108.40.206) is a glycosylphosphatidylinositol–anchored ectoenzyme within the renal proximal tubules (PTs) and is proposed as a diagnostic enzyme of renal disease. We examined the release of RDPase following treatment with CB and various nitric oxide (NO) related compounds in porcine PTs. The RDPase release from PTs was inhibited by CB in a concentration–dependent manner and was also inhibited by sodium nitroprusside (direct NO donor) and L–arginine (NO synthase substrate) in the tested range (0–12 m M). CB–treated (0.1 mg/ml) PTs showed a decreased RDPase activity in comparison with the control group. This inhibition was blocked by 2 m M L–NAME (NO synthase inhibitor) and U73122 (inhibitor of phosphatidylinositol–specific phospholipase C) in a concentration–dependent manner. Eel bile (0–0.1 mg/ml), used as the control, did not significantly affect the RDPase release from PTs. The NO concentration was observed as nitrite, the degradation product of the NO metabolism, increased in proportion to CB and L–arginine. The increase of nitrite to 151.5% by CB treatment (0.1 mg/ml) was blocked by 2 m M L–NAME (95.5%). When the phospholipase C pathway was blocked by 10 and 20 μ M U73122, the nitrite generation decreased to 122.7 and 89.4%, respectively. These results strongly suggest that NO generation and the phospholipase C pathway affect the RDPase release from the PTs and that they may be involved in the development of ARF in vivo following CB ingestion. The release of RDPase from PTs could be a useful tool not only for this CB–caused ARF, but also for the elucidation of other biochemical mechanisms.