1. Single-unit electrical activity has been recorded from fifty-five neurones whose recording sites were located in or immediately adjacent to the marginal zone (lamina I) of the lower thoracic spinal cord (T8-T12) of anaesthetized or decerebrate cats. Their responses to stimulation of somatic and visceral afferent fibres and the sizes of their cutaneous receptive fields have been analysed and compared with the responses and receptive fields of neurones recorded throughout the spinal grey matter. 2. Neurones were classified according to their responses to innocuous stimulation of their somatic receptive fields (i.e. brushing and stroking) or to noxious stimulation (i.e. pinching, squeezing and/or heating above 45 degrees C). 52% of all the neurones recorded in lamina I were driven exclusively by noxious stimulation of the skin (nocireceptive); 33% were driven by both noxious and innocuous stimulation of the skin (multireceptive) and 15% were driven exclusively by innocuous stimulation of the skin (mechanoreceptive). 3. Visceral afferent inputs to these neurones were tested by supramaximal electrical stimulation of the ipsilateral splanchnic nerve (15 V, 0.2 ms, 0.3 Hz). Two types of neurone were distinguished according to their responses to visceral stimulation: (i) somatic neurones, driven only by stimulation of somatic afferent fibres and (ii) viscero-somatic neurones, driven by stimulation of somatic and visceral afferent fibres. Of the neurones recorded in lamina I, 33% were somatic and 67% were viscero-somatic. This proportion was very similar to the percentages of somatic and viscero-somatic neurones recorded throughout the grey matter (37 and 63%, respectively). 4. Viscero-somatic neurones in lamina I had somatic receptive field properties similar to those of viscero-somatic neurones of the entire spinal cord. Half of them were multireceptive, 39% were nocireceptive and 11% were mechanoreceptive. However, somatic neurones in lamina I had receptive field properties different from those of somatic neurones from other laminae: no multireceptive somatic neurones were recorded in lamina I; the vast majority (78%) were nocireceptive and 22% were mechanoreceptive. 5. The majority of somatic and viscero-somatic neurones in lamina I had small somatic receptive fields but, even in this group of cells, viscero-somatic neurones had larger receptive fields than somatic cells. 6. Ascending axonal projections in both dorsolateral funiculi and in the contralateral ventrolateral quadrant were tested in eighteen lamina I neurones. Only one neurone was found to project to the cervical cord.(ABSTRACT TRUNCATED AT 400 WORDS)