Acclimated and acute rates of oxygen consumption of the American oyster Crassostrea virginica (Gmelin) were measured under 9 salinity-temperature combinations during declining oxygen tensions. In acclimated and non-acclimated individuals the basal rate of oxygen uptake increased significantly with each 10 C" rise in temperature. Multiple regression equations relating exposure and experimental temperatures and salinities to VO, indicated: (1) as acclimation salinity decreased, the effect of exposure temperature became more pronounced; (2) as acclimation salinity decreased, the effect of exposure salinity decreased; (3) as acclimation temperature increased, the effect of exposure salinity decreased; (4) as acclimation temperature increased, the effect of exposure temperature increased. There is little or no evidence for temperature acclimation even after 3 wk. Q,, values for warm acclimated oysters were generally higher than those of cold adapted oysters with the exception of individuals acclimated at a salinity of 7 %o S and exposed to 28 %O S. At any experimental salinity the highest Q,,, values were observed between 20 and 30°C. C. virginica is well able to regulate VO, when exposed to declining oxygen tensions at all temperature-salinity combinations tested; this capacity decreased considerably at all test temperatures in oysters acclimated tq 7 % S. There is no clear pattern of response between exposure conditions and ability to regulate VO,; the degree of regulation decreases with increasing temperature and/or decreasing salinity. The results indicate that the respiratory physiology of C. virginica is highly adapted to life in a fluctuating environment.