In newborn rats each retina projects principally to the contralateral superior colliculus, but there is also a sparse projection to the whole of the ipsilateral superior colliculus. During the first 2 weeks postnatally the ipsilateral projection normally becomes restricted to the rostromedial part of the superior colliculus. The restriction of this projection is due to the preferential death of ipsilaterally projecting retinal ganglion cells and is apparently the result of competition between optic fibers from the two eyes, since it can be prevented by enucleation of the opposite eye at birth. To determine if electrical activity plays a role in the normal restriction of the ipsilateral retinocollicular projection, the sodium channel-blocking agent tetrodotoxin was administered to one or both eyes during the first 2 weeks postnatally. Tetrodotoxin blockade of activity in one eye resulted in the persistence of a sparse projection from the opposite eye throughout the ipsilateral superior colliculus and the survival of a substantial number of the ipsilaterally projecting retinal ganglion cells in that eye that would normally have died. When both eyes were treated with tetrodotoxin no restriction of the ipsilateral projection was seen on either side. These findings suggest that the competition between retinal ganglion cell axons (either for terminal space or an essential trophic factor), which normally leads to retinal ganglion cell death and the restriction of the ipsilateral retinocollicular projection, is mediated in some way by electrical activity.