The length of preadult development is negatively correlated with the activity of a majority of studied enzymes, in adult D. melanogaster and D. subobscura flies. This has been shown when activities of seven enzymes (G6PD, 6PGD, aGPD, ADH, HK, ME & IDH) were estimated per mg of protein, or of body mass, in four groups of 6-days old males (50 individuals each, with 3 replications), that had an extremely different preadult development rate. The average activity of studied enzymes is for c. 25 °70 decreased in synchronously grown flies with the longest egg-to-adult development at 21 °C and optimal laboratory conditions, compared with those of the same species with the fastest growth. When the group of slowest growing D. subobscura males (20.4+0.1 days) is compared with the fastest D. melanogaster flies (10.6_+0.03 days), a decrease of 47°70 in enzyme activity was observed. Among studied gene-enzyme loci, four in D. subobscura (aGpd, Adh, Me & Idh) and one in D. melanogaster (Idh) are monomorphic, which implies an involvement of regulatory genes. Among those of D. melanogaster which are polymorphic, specific combinations of alleles have been determined in 'fast' and 'slow' developed flies, suggesting that interactions of structural genes are also of great importance in the control of two studied fitness characteristics.