The mammalian superior cervical ganglion has been the classical preparation for studying cholinergic transmission between neurones. Recently, however, evidence has been presented showing that, in addition to the postsynaptic changes mediated via nicotinic and muscarinic receptors, there is a non-cholinergic component to transmission in this ganglion, as in frog paravertebral ganglia. In the rabbit superior cervical ganglion, Ashe and Libet recorded a late, slow excitatory postsynaptic potential in response to preganglionic nerve stimulation in the presence of nicotinic and muscarinic antagonists. We have found, in the rat superior cervical ganglion, that a postsynaptic biochemical consequence of preganglionic nerve stimulation, namely the acute activation of tyrosine 3-monooxygenase (tyrosine hydroxylase, TH; EC 1.14.16.2), is mediated in part by acetylcholine and in part by a non-cholinergic neurotransmitter. The regulation of this enzyme activity is of particular interest because it catalyses the rate-limiting step in the biosynthesis of the postganglionic neurotransmitter, noradrenaline. In the present paper, we report that the relative importance of cholinergic and non-cholinergic transmission in the regulation of TH activity varies with the pattern of electrical stimulation of the preganglionic nerve trunk.