Rats were treated neonatally with 6-hydroxydopamine. This resulted in a greater than 93% depletion of noradrenaline content of all tissues examined in adult animals. There was a significant increase in the substance P content of semilunar ganglion, L5 dorsal root ganglion and of sciatic nerve. There was a corresponding increase in substance P synthesis as measured by axonal transport in the sciatic nerve. The noradrenaline content and ratio of noradrenaline: substance P in the iris of control animals were 2102 pg/mg and 24 respectively and, confirming the results of other workers, 6-hydroxydopamine treatment caused a doubling of this substance P content. Terminal fields in skin of face and paw had noradrenaline contents of 75 and 41 pg/mg respectively and noradrenaline: substance P ratios of 4.0 and 2.2 respectively and in these areas 6-hydroxydopamine treatment caused a much lesser increase in substance P content. We conclude that the response of sensory neurones to 6-hydroxydopamine is governed by the relative density of the sympathetic and sensory innervations in their terminal areas, possibly reflecting intensity of competition for nerve growth factor.