1. The synaptic connexions established by sprouting of intact preganglionic sympathetic axons were examined by intracellular recording in vitro and by observing the sympathetic end organ responses to ventral root stimulation in vivo. 2. The superior cervical ganglion of the guinea-pig was partially denervated (70-85%) by crushing the cervical sympathetic trunk at the level of the subclavian artery, leaving the ansa subclavia intact. The intact nerve carried some preganglionic axons arising from each of the eight spinal cord segments (C8-T7) contributing innervation to the ganglion. 3. During the first 4 weeks after the operation, there was a two-to threefold increase in the number of steps in the synaptic response elicited in individual ganglion cells by graded stimulation of the ansa subclavia. There was also an increase in the amplitude of the synaptic potential elicited by each preganglionic axon. 4. This increase in the synaptic contribution of the intact nerve to neurones in the superior cervical ganglion after partial denervation was attributed to sprouting of residual preganglionic axons. A major contribution from collateral connexions between ganglion cells was ruled out by intracellular recording form neurones during antidromic stimulation of their axons in the inferior post-ganglionic nerve. 5. After sprouting, the specificity of the sympathetic end organ responses elicited by stimulation of the ventral roots of spinal segments T1 and T4 in vivo was indistinguishable from normal, although the strength of these responses increased from just perceptible acutely after partial denervation to near normal 3-6 weeks after the operation, when sprouting was largely complete. 6. These results show that intact preganglionic axons arising from different spinal levels established selective connexions with different classes of ganglion cells during sprouting.