Glycogenesis and glucose uptake of Hymenolepis diminuta and H. citelli are stimulated by 5% carbon dioxide but less stimulated by 20% carbon dioxide. The stimulatory effect occurs aerobically and anaerobically with H. diminuta. Oxygen (air) has no significant effect on glycogenesis, glucose uptake, or incorporation of glucose-14C into glycogen in H. diminuta, although there was an apparent increase in 14CO, fixation in glycogen under aerobic conditions. The pattern of glycogenesis with time was sigmoid, although glucose uptake in the same experiments was almost linear. The addition of dicarboxylic acids or dicarboxylic amino acids did not serve as a replacement for carbon dioxide in stimulating glycogenesis or glucose uptake. Malate produced no significant effect on the incorporation of glucose-'4C into glycogen. The rate of glucose uptake was dependent on concentration up to about 10 mM. Galactose is absorbed at a high rate but does not serve as a substrate for net glycogenesis. Although considerable data on the carbohydrate metabolism of Hymenolepis have accrued during the past 15 years, some of the gross aspects of metabolism in this worm have not been discussed in the literature. The present paper recounts some experiments dealing with sugar uptake and glycogenesis in Hymenolepis. MATERIALS AND METHODS Hymenolepis diminuta was reared in young, male Holtzman rats, each host being infected with 30 cysticercoids. H. citelli was reared in young adult, male golden hamsters, each host being infected with 15 cysticercoids. After 9 days of growth in the host, food was removed from the hosts' cages for 26 hr, at which time the worms were removed from a group of freshly killed hosts and randomized in samples, usually 23 to 25 worms per sample; this was about 600 mg of worm tissue. Each sample was incubated in 100 ml of test medium, unless otherwise specified, in a shaking 37 water bath. The saline used in all experiments was Ringer's solution, containing 0.025 M tris-maleate buffer at pH 7.4. In experiments with carbon dioxide in the gas phase, sodium bicarbonate was added to the medium at a concentration required to maintain pH 7.4 after temperature equilibration. All solutions were equilibrated with the gas phase for 15 min before initiation of incubation and gassed for the first 15 min of each incubation; they were then sealed by closing a lubricated stopcock and the possibility of leaks in the incubation vessels was monitored by a small manometer attached to each vessel. In experiments with a carbon dioxide-free gas phase, the gas was passed through an alkaline gas scrubber and each vessel contained 2 ml of 20% potassium hydroxide in a center well throughout the incubation period. Received for publication 26 July 1967. * This work was supported by a grant from the NIH, U. S. Public Health Service (AI-01384). Glucose was determined enzymatically with glucose oxidase (Special Glucostat, Worthington) and galactose was determined with galactose oxidase (Galactostat, Worthington). Tests of the specificity of the galactose oxidase preparation showed that it did not react with glucose, mannose, fructose, inositol, ribose, lactic acid, pyruvic acid, succinic acid, malic acid, or fumaric acid, but reacted with ethyl alcohol. Glycogen was determined as the 50% ethanol-precipitable carbohydrate from 30% potassium hydroxide digests of worm samples, using the phenol method of Dubois et al. (1956). In experiments involving the C labeling of glycogen, polysaccharide was extracted with perchloric acid from homogenized worm samples. The glycogen was twice precipitated with 50% ethanol containing 0.2% lithium chloride and washed twice more with 50% ethanol. Aliquots of each sample were analyzed for sugar by the phenol method and for radioactivity by plancheting and counting as solid samples in a gas flow counter.