Guanethidine sulphate 40 mg/kg was administered intraperitoneally daily for 14 days to normal Lewis rats and athymic nude rats of a Lewis background (rnu/rnu). Histological examination of the superior cervical ganglia demonstrated a pronounced chromatolysis of the neurones and a loss of the major part of the nerve cells accompanied by an increased number of small mononuclear inflammatory cells. The extent of chromatolysis and nerve cell death induced by guanethidine did not differ between normal and nude rats, whereas the increase of the number of mononuclear cells was lower in the nude rats than in the normal rats (163 and 268 per cent respectively of the saline treated controls, P less than 0.01). Since guanethidine induced nerve cell death in the T-cell deficient nude rat to the same extent as in normal rats, it is concluded, that the effect is caused by either a thymus-independent immune-response or by a direct toxic effect.