The age-dependent effects of sympathetic superior cervical ganglionectomy on smooth muscle alpha-adrenoceptor contractile responses were investigated in Müller's muscle in the superior eyelid of the Sprague-Dawley rat. Ganglionectomies in juvenile (32 days) and adult (70 days) rats produced sustained sympathetic denervation, determined by absence of contractile responses to electrical stimulation of ipsilateral and contralateral sympathetic chains and reduced responses to indirect release of nerve terminal norepinephrine by tyramine. Denervation was associated with diminished contractile responses and postjunctional supersensitivity to alpha-adrenoceptor stimulation by methoxamine. In contrast, contractile responses in rats ganglionectomized as neonates (day 3) were greater than those of other groups and sensitivity to methoxamine was comparable to control values. Responses to tyramine suggested that noradrenergic reinnervation had occurred. Electrical stimulation of the ipsilateral chain did not elicit contractions; however, stimulation of the contralateral chain evoked contractions in all neonatally ganglionectomized preparations and responses were present by 20 days of age. Contralateral reinnervation was derived from postganglionic sympathetic neurons with axons in the internal carotid nerve. This pathway is probably not formed by collateral sprouting of neurons with bilateral projections at the time of surgery for contralateral stimulation did not evoke contractions in the neonate. It is concluded that Müller's muscle is reinnervated by sympathetic neurons in the contralateral chain after denervation in neonates but not in older animals, and it is suggested that the higher potential for plasticity is attributable to the immaturity of the neonatal sympathetic nervous system.