The autonomic innervation of the rat testicular capsule has been investigated pharmacologically and histochemically. Electrical field stimulation of isolated capsules produced contractile responses which were abolished, or largely depressed, in the presence of bretylium (2 to 4× 10−6 g/ml). Any residual response was abolished in the presence of hyoscine (5 × 10−7 g/ml). No evidence for the presence of a relaxatory innervation was obtained. Fluorescence histochemistry demonstrated the existence of a dense plexus of fine varicose fibres exhibiting catecholamine fluorescence in the mediastinal region, and of more scattered fluorescent fibres in other areas. The blood vessels in the region of the rete testis also received a dense innervation by fluorescent fibres. Fibres which stained heavily for acetylcholinesterase were sparsely distributed in the mediastinal and non-mediastinal regions of the capsule, but were absent from the blood vessels. It is concluded that the capsule receives both adrenergic and cholinergic motor nerves, the adrenergic supply predominating, and that acetylcholinesterase positivity can be used as a criterion of cholinergic autonomic nerves in the rat, at least with the preparative conditions employed by us.