Isolated rabbit corpus cavernosum relaxed in response to ultraviolet (UV) light (365 nm). The UV light-induced relaxation (photorelaxation) was diminished on repeated UV irradiation from 30.5 +/- 4.0% (the first photorelaxation) to 15.5 +/- 2.7% (the last photorelaxation). Hydroxocobolamine of 100 muM and hemoglobin (Hb) of 10 muM, which are nitric oxide (NO) scavengers, and 10 muM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a soluble guanylyl cyclase inhibitor, markedly reduced photorelaxation, However, 300 muM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO) failed to inhibit photorelaxation. NaNO2 and N-G-nitro-L-arginine (L-NA) but not 3-nitro-L-tyrosine (3-NT),were found to be photosensitive in that these compounds are photolysed to release NO, as demonstrated by use of an amperometric NO probed NO signals produced by 500 muM NaNO2, and 500 muM L-NA were 133.3 +/- 28.9 and 54.4 +/- 10.4 pA, respectively, Not 3-NT but the other compounds (all 200 muM) also enhanced photorelaxation of the cavernosal tissue. Based on these findings, the substance, which mediates photorelaxation, could be NO released from putative stores in the rabbit corpus cavernosum, and L-NA as well as NaNO2 but not 3-NT produce NO under the influence of UV light. (C) 2002 Published by Elsevier Science B.V.