Action potentials of single ganglion cells were recorded with microelectrodes in excised and opened frog (Rana temporaria L.) eyes before and after picrotoxin application. Area threshold curves were measured both before and after drug application. The drug decreased or abolished the lateral inhibition observed when the cones and green rods were stimulated in the inhibitory receptive fields. In the response pattern interruption often occurs after the first action potentials. Picrotoxin shortened this pause and lengthened the subsequent train of action potentials, i.e., the cell responded with smooth and sustained discharges after picrotoxin application. Picrotoxin also temporarily decreased the sensitivity of the green rod-mediated signals from the excitatory receptive field. Picrotoxin decreased red rod-mediated sensitivity in the completely dark-adapted retina. After an 1-1.5% bleach of the rhodopsin in the dark-adapted eye, the early course of dark-adaptation was similar with and without picrotoxin but after drug application the adaptation curve levelled off at a higher threshold. The organization of the receptive field is discussed in relation to the anatomy, electrophysiology and histochemistry of the cells transmitting the signals from the receptors to the ganglion cells. The role of gamma aminobutyric acid as an inhibitory transmitter in the horizontal and amacrine cell layers is also considered.