The administration of guanethidine to adult rats was shown by morphological criteria to destroy sympathetic neurons. Using biochemical and functional criteria the degree and permanence of this sympathectomy was evaluated. Young adult male rats (260-300 g) were injected with saline (controls) or with guanethidine for 5 wk. The status of the sympathetic nervous system in the animals was evaluated 1, 3 and 6 to 7 mo. after cessation of treatment. Seven mo. after cessation of treatment the activity of tyrosine hydroxylase in the superior cervical ganglia of treated animals was greatly reduced, as were the norepinephrine levels in peripheral tissues. The concentration of epinephrine and the activity of tyrosine hydroxylase in adrenals were not different from controls at any of the times studied. Norepinephrine concentrations in several areas of the CNS were unchanged. Increases in blood pressure in response to stimulation of the sympathetic vasomotor outflow in the pithed rat preparation were markedly and permanently reduced in guanethidine-treated animals. Isolated intestinal nerve-muscle preparations from guanethidine-treated animals usually contracted in response to nerve stimulation, rather than relaxing as in controls. The response to stimulation of the hypogastric nerve in vas deferens preparations was reduced 1 mo. after cessation of treatment. The responses of the vas deferens from guanethidine-treated and control animals were the same 7 mo. after treatment despite a 93% reduction in norepinephrine concentration. The administration of guanethidine to adult rats produces a marked and permanent destruction of the peripheral sympathetic nervous system.