Several results emerge from this study which, it is believed, should assist in defining the problem and, at the same time, should suggest types of investigation which are most likely to contribute to an understanding of metabolism in Drosophila melanogaster. It has been shown that a close approach to a knowledge of metabolism in Drosophila pupæ cannot be made if sole dependence is placed on weight of respiring tissues as a guide. In a very rough way, pupal weights show a correspondence with the trends of metabolic rates, but, for particular matings, especially when the larvae are maintained at lower temperatures, the correspondence is largely lost. It has also been shown that rates of oxygen consumption are very irregular and vary in a manner highly suggestive of the influence of environmental agencies. The not infrequently pronounced elevations above the general level of metabolism lend support to the conclusion that the metabolic rates for comparable samples of respiring tissue are not necessarily fixed within narrow limits. Finally, it has proved impossible to find a metabolic difference between the sexes or to detect any difference in metabolism between inbred and noninbred stocks.