Incubation of cat retinas with 3H-glycine in vitro, followed by horizontal sectioning and autoradiography, showed labeling of 10-12% of bipolar cells and 45% of amacrine cells. To ascertain the effects of glycine-accumulating bipolar and amacrine cells on the response properties of retinal ganglion cells, in vivo iontophoretic studies were performed in the cat eye. Glycine inhibited all ganglion cells, and this action was blocked by strychnine. Aminophosphonobutyric acid (APB) suppressed ON-ganglion cells, but activated OFF-ganglion cells. The influence of APB upon OFF-ganglion cells could be completely blocked by strychnine. In the mudpuppy, APB suppressed ON-bipolar cells without affecting OFF-bipolar cells and without direct effects on other retinal neurons (Slaughter and Miller, 1981, 1985). If APB has the same mode of action in the cat retina, the present results suggest a glycinergic inhibitory input of the ON-channel upon OFF-ganglion cells.