A method of selectively blocking the larger nerve fibres in the optic nerve of the cat is described. The optic nerve is compressed between a small balloon and the inside of a small metal cylinder. The block is monitored by stimulating and recording on opposite sides of the blocked region. The t1 (Y) response can be eliminated usually with some loss of the t2 (X) response. Stimulation of the blocked optic nerve initially excites certain cells (r2 or X) in the dorsal lateral geniculate nucleus (d.l.g.n.) but this is followed by an inhibition of both r1 (Y) and r2 (X) cells. This inhibition is similar in time course to that produced by stimulation of t1 fibres, suggesting that the effect may be mediated via a similar mechanism. The effect is present in a preparation in which all t1 fibres have degenerated, removing the possibility that it might have been due to inadvertent stimulation of t1 fibres. Experiments on a small number of r2 (X) d.l.g.n. cells have confirmed the inhibitory effect of stimulation of the smaller optic nerve fibres on X cells.