Retinas from goldfish, chicken, rate, guinea-pig, rabbit and humans were exposed to [3H]isoguvacine either by intravitreal injection in vivo or by incubation in a balanced salt solution. The distribution of radioactivity was then studied by autoradiography. The substance labelled a set of presumed amacrine cells in all types of animals. The inner plexiform layer was well demarcated and a variable number of ganglion cells was marked. In goldfish and chicken, radioactivity could also be seen in horizontal cells, particularly when the retina had detached from the pigment epithelium. Even 24 hr after an intraocular injection there was a significant amount of radioactivity left in nerve cells. Only a little glial labelling could be seen at any time. Since the distribution of labelled neurones was similar to that of GABA neurones and since isoguvacine is a potent GABA agonist, it seems reasonable to presume that [3H]isoguvacine labels GABA neurones. The uptake is predominantly neuronal with strong binding to presumed GABA storage sites.