Seasonal fluctuations of oxygen uptake, ammonia excretion and O/N ratio were measured in the predatory gastropod Concholepas concholepas. Oxygen uptake was higher in spring, coincident with the active gametogenic processes in C. concholepas. Temperature was lower in spring than in summer suggesting that the reproductive condition played the most important role in oxygen uptake. Ammonia excretion was at a maximum in the spring. Minimum values were observed during autumn, a season of low reproductive activity and decreasing temperature. The energy utilized in oxygen uptake and ammonia excretion is significantly different depending on the season, being an important factor which can affect the overall fitness of the animal.