ABSTRACT Isolated photophores of Porichthys exposed to u.v. light (365 nm) exhibit a green fluorescence localized in the photocytes and a bluish fluorescence originating from the lens-like body. The luminescence of photophores evoked by epinephrine, nor-epinephrine or potassium cyanide decreases with the intensity of the green fluorescence of the photogenic tissue; the bluish fluorescence of the lens-like body does not chnge significantly. The total amount of light emitted is a linear function of the decrease of the greenish fluorescence intensity. The slope of the regression line is maximal in response to epinephrine or nor-epinephrine 5·10−4 M and potassium cyanide 10−3 M. It decreases significantly for higher concentrations. The decrease of fluorescence observed during the light emission is tentatively explained by oxidation of a luciferin present in a fluorescent form in a fresh photophore.