A specific fluorescence method has been used for the cellular demonstration of biogenic monoamines in the nervous system of Nephtys. The central nervous system contains both green fluorescent (adrenergic) and yellow fluorescent (5-hydroxytryptaminergic) fibres and neurons, the neurons showing a repetitive pattern in successive segments in the nerve cord. The proportion of yellow neurons is much higher in the ventral nerve cord, while the green neurons predominate in the supra-oesophageal ganglion. Many of the fluorescent fibres in the neuropile are beaded. Three of the four segmental nerves carry fluorescent fibres peripherally. Except for yellow neurons in the wall of the intestine, all peripheral fluorescent cells are green. Of these, a few in each segment are located in peripheral ganglia, and the rest resemble the bi-polar epidermal sensory cells found in other polychaetes. The only yellow fibres visible peripherally are the multiple varicosities which occur at points of muscle insertions on basement membranes. It is possible that these are the end plates of a set of motor fibres. On the basis of the distribution and morphology of structures exhibiting the two types of fluorescence it is postulated that the green system may have a sensory and the yellow system a motor function.