In order to evaluate seasonal changes in respiration rates of the Peruvian scallop Argopecten purpuratus, oxygen consumption was measured in situ for three ranges of sizes (R1: 30 - 40mm, R2: 45 - 55mm and R3: 80 - 90 mm) in Atenas beach (Paracas Bay) during February, March and August 2007. Four to six acrylic chambers connected to submersible pumps were installed to set an opened/closed recirculating system. Once the scallops were acclimated, initial water samples were taken with the opened recirculation system, after that, the closed recirculating system was set and one hour later final samples were taken. Respiration rates (mg 02 h-1) appeared to be directly related with size of individuals as its shown in the August results, with values of 0.810, 1.677 and 4.480 for R1, R2 and R3 respectively. Results also showed significant differences (P<0.05) between March and August for the three range of sizes showing the influence of seasonal changes on the respiration of the scallops. Correlation analysis shows a positive relation between respiration rates with size (r=0.548) and respiration rates with weight (r=0.671). There was also a positive correlation between respiration rates and temperature (r=0.442). An inverse correlation was observed between oxygen concentration and the respiration rate (r=-0.471).