1. The oxygen consumption and surface area of individual diploid and triploid prepupae of Drosophila melanogaster have been measured, the cells of triploid animals being larger.
2. The mean weights for the types examined are different but their ranges overlap almost completely. By covariance analysis it is shown that, after adjustment for difference in body size, there are no differences in the rates of oxygen consumption. It is concluded that, for these animals, cell size has no influence on the rate of oxygen consumption.
3. The relationships between body weight, surface area, and oxygen consumption have been further investigated. It is shown that, despite the greater inaccuracy of the method by which surface area is determined, oxygen consumption can be predicted more accurately from surface area than from body weight.
4. The results are discussed in relation to an earlier investigation of the oxygen consumption of other genotypes (Ellenby, 1945 a, b). Possible technical causes of certain differences between the two series of results in the relationship of oxygen consumption and body weight are explored; it is concluded, however, that they are almost certainly due to differences, not necessarily genetical, between the animals used in the two series.