The nereidid Nereis (Neanthes) virens undergoes drastic behavioural, morphological and physiological changes during its sexual maturation (epitoky). This metamorphosis prepares benthic worms for a brief pelagic existence devoted to mating although in N. virens only mature males leave their burrows to swarm. After spawning, individuals of both sexes die. Specific adjustments of energy metabolism pathway allowing higher muscular activity and swimming capacity remain to be eluded. This study compared atokous worms (immature) and epitokous (mature) swimming males and benthic females of N. virens to detect metabolic changes that could occur during epitoky. Epitokous males showed significantly higher electron transport system, citrate synthase and aspartate aminotransferase activities (p<0.01) and significantly lower lactate dehydrogenase activity (p<0.01) compared to atokous worms and epitokous females. There was no difference in antioxidant enzyme capacities between epitokes and atokes. Lipase and trypsin activities were significantly lower (p<0.01) in epitokous males. The enzymatic changes observed are likely related to the metabolic adjustments required to support higher swimming abilities. Maintenance of antioxidant capacities could be related to protection of germinal tissues more than long term survival, since N. virens die after spawning.