The histological visualization of choline acetyltransferase (CAT) and acetylcholinesterase (AChE) on frozen sections of prostomia of Nereis virens indicate a concentration of cholinergic activity in the anterior brain. Components are probably sensory epithelial cells with cholinergic axons entering the brain in cephalic nerves and efferent cholinergic axons to prostomial muscle leaving the brain in the same nerves. There are also subepidermal cholinergic cells that may be second order motor neurons serving epidermal mucous cells. The smaller, second lobe of the corpora pedunculata and its associated vertical fibre tract are CAT(+) and appear continuous, on each side of the cerebral ganglion, with a dorsal and ventral longitudinal bundle of AChE(+) fibers. This system tapers to nothing at the level of the posterior eyes. There is a small AChE(+) component to each optic nerve and AChE is present in the nuchal epithelium. These observations are discussed in relation to earlier studies on aminergic and neurosecretory activity in the same ganglion.