When stimulated in some suitable manner Chaetopterus secretes a luminescent material into the surrounding sea water. The luminescent secretion is discharged fromglands which are widely dispersed over the surface of the animal; the most conspicuous are two glandular patches on the dorsal surface of segment XII. Secretory materialis forced out of the cells by some contractile process; discharge is not merely theresult of secretion pressure. Under repetitive stimulation the intensity of the luminescent response decreases owing to the intervention of fatigue. Fatigue has been interpreted as a gradual exhaustion of luminescent material in the glandular cells (Nicol, 1952b,c).The present investigation seeks to analyse the onset and progress of fatigue in greater detail.