1 The ability of intravenous L-DOPA to block sympathetic and parsympathetic nerves has been studied in cats and dogs pretreated with a monoamine oxidase inhibitor.2 L-DOPA inhibited positive chronotropic and pressor responses to dimethylphenylpiperazinium (DMPP) and McN-A-343 in dogs, and contractions of the nictitating membrane produced by these ganglion stimulants in cats.3 Responses of the cat nictitating membrane to preganglionic stimulation were inhibited by L-DOPA to a greater extent than those to postganglionic stimulation of the cervical sympathetic chain.4 In dogs, L-DOPA had no vagolytic action, but depressed vasoconstrictor responses elicited in the perfused hind-limb by electrical stimulation of the lumbar sympathetic chain.5 The degree of lumbar sympathetic chain inhibition correlated with the pressor response following L-DOPA, and both effects were prevented by prior decarboxylase inhibition.6 These results suggest that the decarboxylation products of L-DOPA do not impair parasympathetic nerve activity but depress sympathetic nerve function predominantly by inhibiting both muscarinic and nicotinic sites of sympathetic ganglia.