Abdominal photophores isolated from Maurolicus muelleri freshly collected in the Strait of Messina show a stable oxygen consumption rate of 149.8 +/- 10.1 nmoles g-1 min-1 (N = 31). 2. In the presence of adrenalin (5 X 10(-4) mol/l), the photophores that did not luminesce showed a 78% decrease of the resting oxygen uptake. When the photophores did luminesce, the oxygen consumption never increased above the resting level, instead, it decreased at a different rate according to the amount of light emitted. 3. Photophores pretreated with MnCl2 (40 mM) and stimulated with adrenalin (5 X 10(-4) mol/l), showed a rapid and large emission of light associated with a significant increase of the oxygen consumption above the resting level. 4. The results are discussed with reference to the control mechanism of luminescence in mesopelagic and epipelagic fish.