Mammalian sympathetic neurons in vivo may express either a noradrenergic or cholinergic phenotype. In view of the opposing effect of noradrenaline and acetylcholine on most autonomic target organs, the target-appropriate expression of neurotransmitter is critical. We have examined the maturation of the sympathetic innervation of rat sweat glands to define the developmental mechanisms regulating neurotransmitter choice in vivo. Eccrine sweat glands and their sympathetic innervation develop together postnatally in the rat. Early postnatal innervation expresses only noradrenergic properties, but as the glands and their innervation mature, noradrenergic properties decrease dramatically and cholinergic features appear in the same population of neurons. To investigate the role of the sweat gland in this change we have used a transplantation paradigm which allows sweat glands to be innervated by sympathetic neurons that would normally innervate noradrenergic target organs and remain noradrenergic throughout life. We observe that the sympathetic neurons that innervate the novel cholinergic target alter their neurotransmitter properties and develop a cholinergic phenotype. These results indicate that target organs are able to induce appropriate neurotransmitter traits in the neurons that innervate them.