Intersegmentally evoked primary afferent depolarization (PAD) was analysed to investigate whether any lower lumbar propriospinal neurones are involved in mediating PAD from group I afferents to group I afferents both in the same segments and in Clarke's column. The intersegmental PAD of lower lumbar afferents, as judged by recording dorsal root potentials, was evoked by stimuli applied in the grey matter of L3 and L4 segments. With intraspinal stimuli of 10 microA or less PAD was evoked from two foci: from within the middle part of the dorsal columns and from the dorsal part of the dorsal horn. Dorsal root potentials evoked from the dorsal horn focus appeared with longer latencies. When the dorsal columns were transected PAD was evoked only from the dorsal horn focus. No PAD appeared upon stimulation of Clarke's column after transection of the dorsal columns even with stronger (20 microA) stimuli. Interactions between the actions of the intraspinal stimuli and of different groups of afferents were analysed to define the neuronal pathways via which the intersegmental PAD was evoked. Neurones located within both the lower and the upper lumbar segments were found to be involved. Indications have only been found for a contribution of neurones mediating PAD from afferents other than group I afferents. Lesions of the ipsilateral and contralateral, lateral and ventral funiculi (in addition to the dorsal columns) were made in order to define which of these funiculi are required for the appearance of the intersegmental PAD. The intersegmental PAD could be evoked from the dorsal horn when either the contralateral or the ipsilateral funiculi were left intact.