Abstract Afterpositivity elicited in the dorsal nucleus of the lateral geniculate body of cats by stimulation of either of the optic nerves was found to be closely associated with depression of the postsynaptic spike and afterpositivity elicited by a test stimulus to the other optic nerve. The radiation spike was found to be more vulnerable to the inhibitory effect of afterpositivity than the postsynaptic spike recorded in the lateral geniculate body (LGD). Anatomical and electrophysiological evidence is adduced in support of the hypothesis that much of the afterpositivity is not a positive afterpotential but a hyperpolarizing postsynaptic potential propagated among principal geniculate neurons by neurons of the short axon type which are activated by recurrent collaterals from radiation axons.