Superior cervical ganglia were removed unilaterally from 6 adult cats. 2 animals were sacrificed at 2, 4 and 7 d and the innervation of the canine apex examined quantitatively. The number of non-myelinated axons was lower on the operated side 4 and 7 d after sympathectomy. The number of degenerating axons was always greatest on the operated side and highest 2 d after operation. By combining the proportion of axons lost with those in the process of degenerating it is estimated that between 4.8 and 12.5% of the axons (52-88 axons) entering the apex of the cat's canine are sympathetic. Degenerating axons were often within the same Schwann cell as intact, presumably sensory, axons and were sometimes in contact with them. This contact may be the anatomical basis for the modulation of nociceptive input by sympathetic activity.