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Here we report that the active component of Viagra, Sildenafil and the first metabolite, N-desmethyl-sildenafil (UK-103, 320) increased the amplitude of flash-evoked electroretinogram (ERG) of dark-adapted albino rat retina. Effects of Sildenafil and N-desmethyl-sildenafil were comparable to those of the known phosphodiesterase inhibitor, Zaprinast. The photoreceptor cell response was isolated by blocking the glial K+ ion-buffering and the on-bipolar components of the ERG with the use of BaCl2 (500 μM) and the specific type VI metabotropic glutamate receptor agonist, dl-2-amino-4-phosphonobutyric acid (25 μM), respectively. Zaprinast, Sildenafil and N-desmethyl-sildenafil (1 μM each) increased the amplitude of photoreceptor cell response either. Besides, Sildenafil was significantly more effective than N-desmethyl-sildenafil. These findings suggest an increased sensitivity of photoreceptor cells in the presence of Sildenafil and it is metabolite.