1. The effects of dopamine on isolated rabbit superior cervical ganglion were investigated with intracellular recording techniques.2. Dopamine (10(-5)-10(-3)M) depressed the amplitude of the excitatory post-synaptic potential (e.p.s.p.) and blocked impulse transmission.3. Dopamine (10(-4)M) induced a slight (2-5 mV) post-synaptic hyperpolarization without altering membrane conductance.4. The post-synaptic membrane sensitivity to acetylcholine (ACh) applied iontophoretically was not affected by dopamine.5. Dopamine decreased the frequency of miniature excitatory post-synaptic potentials (m.e.p.s.p.s) in a high K(+) solution, with no change in the amplitude of m.e.p.s.p.s.6. Dopamine reduced the quantal content of the e.p.s.p. in a low Ca(2+) and high Mg(2+) solution, but had no effect on the quantal size.7. The ganglionic blocking effect of dopamine was antagonized by phenoxybenzamine, but not by propranolol.8. The results show that the ganglionic depressant effect of dopamine is exerted primarily through an alpha-adrenoceptive site at the presynaptic nerve terminal.