The glycogen stores of adult Schistosoma mansoni worms could be labelled by incubation of the worms, after an initial reduction of their glycogen content, in the presence of [6-14C]glucose. Subsequent breakdown of the labelled glycogen by the parasite revealed that glycogen was degraded to lactate and carbon dioxide. The degradation of glycogen, as compared to that of glucose, resulted in slightly different ratios of these two end-products. This indicates that glycogen breakdown did not replace glucose breakdown to the same extent in all cells and that Krebs-cycle activity was not uniformly distributed throughout the cells of this parasite. Both fructose and mannose could replace glucose as an energy source and the rate of glycogen synthesis from either of these two carbohydrates was higher than from glucose. No indications for glyconeogenesis from C3-units were found. Glycogen metabolism of S. mansoni was not influenced by hormones of the mammalian host. It is regulated by the external glucose concentration and by the level of the endogenous glycogen stores. Studies on paired and unpaired worms showed that no interaction between male and female was necessary for the synthesis of glycogen by female worms.