Using an in vitro preparation of the cat dorsal lateral geniculate nucleus, we have studied the passive membrane properties and the electrotonic structure of single cells each identified as X or Y on the basis of their morphological features following intrasomatic injection of horseradish peroxidase. The input resistance of X-cells is higher and the membrane time constant longer than of Y-cells. The electrotonic length and the dendritic to somatic conductance ratio are similar for both classes of neurones.