Autoradiography with 3H-adenosine was used to localize cells that accumulate adenosine in the cat retina. Electrophysiologic effects elicited by adenosine on DC-electroretinograms (ERG) and optic nerve responses (ONR) were studied in isolated, arterially perfused cat eyes. Subpopulations of cells localized in the ganglion cell layer and inner nuclear layer showed clear labeling for adenosine. This purine nucleoside enhanced the ERG b-wave and the standing potential; depressed the light peak; and markedly depressed the ONR, in which it reduced the amplitudes of the ON-, plateau-, and OFF-components. A vasodilatory action of adenosine was documented by an increase in perfusion flow rate. Our data suggest that adenosine in cat retina has complex modulatory effects, involving the retinal pigment epithelium, neuronal structures, blood vessels, and probably glial cells.