The effects of 6-hydroxydopamine (6-OHDA) treatment of neonatal and adult rats on vasopressor responses to postsynaptic alpha-adrenoceptor activation were studied in adult pithed rats. The pressor response to electrical stimulation of the thoracosympathetic outflow (T7-T9) was markedly reduced after neonatal or adult 6-OHDA treatment, indicating a functional destruction of the sympathetic input to the vasculature. Pressor response curves to injected noradrenaline (a nonselective alpha-agonist) and phenylephrine (a selective alpha 1-agonist) were shifted to the left in all 6-OHDA-treated animals. However, the pressor response to the selective alpha 2-agonist M-7 (5,6-hydroxy-2-dimethylamino tetralin) was unaltered in all the 6-OHDA-treated animals. It is concluded that functional destruction of peripheral sympathetic nerve endings in vascular smooth muscle by treatment of neonatal or adult rats with 6-OHDA produces an increased pressor response to alpha 1-adrenoceptor agonists, but not to alpha 2-adrenoceptor agonists in the pithed rat.