Microelectrode recordings were made from dorsal column fibres at the 12th thoracic segment in cats. Two kinds of activity were elicited by electric shocks applied on the sciatic nerve in the popliteal fossa: primary afferent activity and trans-synaptically evoked activity. The dorsal column postsynaptic (DCPS) fibres represented 9.3% of the total population of fibres studied in the fasciculus gracilis and 14.5% of those with receptors in the skin. They were found to lie between the primary fibres of cutaneous and those of deep origin. The fastest fibres of the alpha range contributed to their activation and it is likely that C fibres contributed as well. 87% of the DCPS fibres studied at Th 12 were antidromically activated from the first cervical segment, and their conduction velocities measured between cervical and thoracic levels ranged from 16 to 71 m/sec.