The effect of noradrenaline on the hypogastric ganglion of the guinea pig was studied to establish the role of catecholamine-containing sites in the sympathetic ganglia with the hypogastric nerve-vas deferens preparation. In these experiments the ganglion was separated pharmacologically from the muscle by a partition. An application of noradrenaline, adrenaline and isoproterenol each to the ganglion inhibited a twitch response of the vas deferens to preganglionic nerve stimulation at low frequencies. The drug effect decreased with increasing frequencies of stimulation. The inhibitory action of these drugs was antagonized by phentolamine, but not by propranolol. Clonidine also reduced the response to the nerve stimulation in a similar manner as did catecholamines, and the inhibitory action was antagonized by phentolamine. In contrast, phenylephrine had no significant action. The inhibitory effect of noradrenaline and clonidine was antagonized by the alpha-2 antagonist yohimbine, but not by such alpha-1 antagonists as prazosin and E-643. Noradrenaline did not affect the ganglion-stimulating action of acetylcholine. These results suggest that the inhibitory action of noradrenaline may be mediated by alpha-2 adrenoceptors on presynaptic cholinergic nerve endings of the guinea-pig hypogastric ganglion. These data are discussed in relation to the presence of noradrenaline-containing sites in this ganglion.