Photorelaxation of arteries by ultraviolet (UV) light is hypothesized to result from nitric oxide (NO) released from photoactivable stores. Recently, a study reported enhanced photorelaxation of aortic tissue from rats administered the NO synthase (NOS) inhibitor N<sup>ω</sup>-nitro- L-arginine ( L-NNA). Presumably, the potentiated photorelaxation was due to NO generated from UV-light-induced decomposition of the NO<sub>2</sub> moiety of L-NNA. However, we hypothesized that photorelaxation is: (1) not the result of NO synthesis and subsequent activation of guanylate cyclase and (2) not due to hyperpolarization induced by NO or any other factor. Endothelium-denuded rat aortic rings were suspended in isolated baths for isometric force measurement. Rings were exposed to UV light (366 nm) before addition of phenylephrine or KCI, and then at each agonist concentration during a cumulative concentration response curve. NOS inhibition by L-NNA and L-thiocitrulline, which lacks an NO<sub>2</sub> group, enhanced photorelaxation of basal myogenic tone and contraction to phenylephrine (EC70). Furthermore, relaxation of a maximum phenylephrine-induced contraction to the NO donor S-nitroso-N-acetyl-D L-penicillamine during UV light exposure was not altered by incubation of rings with L-NNA or tissues from animals fed L-NNA. These data demonstrate that NO is not produced endogenously or from the breakdown of L-NNA to result in photorelaxation. Methylene blue (MB) did not alter photorelaxation, suggesting that cGMP is not essential to the response. MB and L-NNA together potentiated photorelaxation of basal myogenic tone and phenylephrine-induced contraction. Photorelaxation of KCl-induced contraction was unaltered, indicating that hyperpolarization does not contribute to the relaxation. Photorelaxation of basal myogenic tone and KCl-induced contraction excludes the possibility that UV light is interfering with agonist-receptor binding. Collectively, these results refute the hypotheses that photorelaxation results from activation of the NO-cGMP pathway, release of a hyperpolarization factor, or inhibition of drug-receptor interaction. Interestingly, photorelaxation may be inhibited by NO-cGMP pathway activation, uncovering a novel effect of this messenger system on vascular reactivity.