The tissue activities of the oxidative pentose shunt enzymes, glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49) and 6-phosphogluconate dehydrogenase (E.C. 1.1.1.44), in the larvae of Drosophila melanogaster are not dependent on the amount of flux through the oxidative pentose shunt pathway. An oxidative pentose shunt deficiency effects about a 40% reduction in the NADPH concentration in early third instar larvae, resulting in a six-fold difference in the NADPH/NADP+ ratio between wild-type and pentose-shunt-deficient larvae. The capacity of pentose-shunt-deficient larvae to synthesize triglyceride in response to a high concentration of dietary sucrose is only 73% of the wild-type level. Environmental temperature influences on the fatty acid composition of larvae are not altered by an oxidative pentose shunt deficiency.