1. The oxygen consumption of males of Artemia ranging in length from 3.27 to 7.25 mm was measured in animals reared in sea water (S‰35) and brine of salinity 140‰. 2. The ratio of dry weight to wet weight was meausured over the size range. Males reared in sea water have a higher water content than in more concentrated brine. Large males contain relatively more water than small in both concentrations of brine. The increase in water content with increasing size occures at the same rate in the two media. 3. The area of the second antennae of males increases more rapidly with increasing size in animals reared in sea water than in concentrated brine. 4. Total oxygen consumption is proportional to the 0.883 and 0.624 power of the dry weight for animals reared in sea water and in brine of salinity 140‰ respectively. 5. The increase in oxygen consumption with increasing size of males is significantly greater in sea water than in more concentrated brine. This is related to the larger area of the second antennae in sea water. 6. The oxygen consumption of males is compared to that of females. In concentrated brine there is no difference in the magnitude and rate of oxygen uptake of males and females. In sea water the oxygen consumption of males increases more rapidly than females with increasing size of the animals.