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o 1. Isolated photophores of the living bathypelagic fish Argyropelecus hemigymnus respond to a strong (25–80 V) and long (4–15 msec) electrical stimulus by a brief flash emission (F response). 2. Weaker and shorter stimuli are subthreshold, but when repeated at a frequency equal to or higher than one per second, the photophores respond by a slow light emission (S response); it is characterized by a long emission latency time, at least 2 sec duration and by a progressive increase in the rate of light production during the stimulation period. The maximal amplitude of the response is more than 300 times lower than the response of the photophore of the epipelagic luminescent fish Porichthys. 3. A train of strong (35V) and long stimuli (4 msec) applied at high frequency (100/sec) quench temporarily the luminescence of spontaneous luminescent photophores. The inhibitory response is reversible: after the end of stimulation, the light production returns to the level recorded before the stimulation. 4. The inhibitory response to electrical stimulation is tentatively explained either by the presence of an inhibitory nervous system or by the liberation of an excess of adrenalin, the possible natural neuromediator that inhibits luminescence of isolated photophores at high concentration.