The applications of ultraviolet (UV) light (336 nm) on the upper oesophageal strips of frog elicited relaxant responses in the presence of NaNO2 (50 mu M). The tissues were mounted under the tension 0.5 g in an organ bath containing Ringer solution, maintained at 25 degrees C and gassed with 100% O-2. The responses were recorded on a kymograph via an isotonic lever. Antimegaloblastic agents, including hydroxocobalamin (1, 10, and 100 mu M), cyanocobalamin (1, 10, 25, and 100 mu M), and folic acid (1, 10, 50, 100, and 200 mu M), significantly attenuated the relaxation response to UV light. Folinic acid (1, 10, 25, and 100 mu M), however, enhanced the relaxation. Pyrogallol (50 mu M), hydroquinone(50 mu M), and diethyldithiocarbamic acid (8 mM) were found ineffective for attenuation, though FeSO4 (200, 400, and 500 mu M) and hemoglobin (50 mu M), respectively, exerted significant inhibition. L-arginine methylester (500 mu M) did not impair UV-induced relaxation. Based on these results, we concluded that a mechanism involving undefined action(s) of antimegaloblastic drugs may cause alterations in the UV light-induced relaxation of the tissue used.