1. Conduction velocities of two types of on-centre monkey ganglion cells, called phasic and tonic, have been measured by stimulating their axons in the optic tract while recording from their cell bodies in the retina.2. The average conduction velocity of twenty-two phasic and twenty-seven tonic cells is 3.8+/-S.D. 0.6 and 1.8+/-S.D. 0.4 m/sec respectively. Since the latter, but not the former, show opponent-colour responses, retinal signals carrying information about colour appear to be travelling in smaller axons than those not handling such information.3. Stimulation of the optic tract elicits several graded intraretinal potentials, which are negative in the optic nerve fibre layer and positive in the inner plexiform layer. One of these potentials, which is largest near the fovea, occurs simultaneously with the antidromic impulses of tonic ganglion cells and is considered to result from extracellular current generated by these cells.4. Stimulation of the optic tract suppresses the orthodromic responses of ganglion cells, more for phasic than for tonic ones. This suppression is only observed after a cell is antidromically driven and is considered most likely due to a transient hyperpolarization of the cell's membrane potential following an impulse.