Under in vitro anaerobic and aerobic incubations the presence of 5-hydroxytryptamine (5-HT) resulted in an accelerated rate of carbon transfer through the carbohydrate metabolic pathways of Hymenolepis diminuta resulting in an increase in worm tissue total carbon pools. Under aerobic incubation, levels of phosphoglycerates, phosphoenolpyruvate (PEP), and pyruvate were significantly increased over the corresponding levels under anaerobic incubation. Excreted end products of carbohydrate metabolism also differed significantly depending on the gas phase and the presence of 5-HT. Under atmospheric (21%) oxygen concentrations excreted levels of lactate, succinate, and acetate were all significantly elevated; under a 95% O2-5% CO2 gas phase only lactate excretion was increased, and under a 95% N2-5% CO2 gas phase only succinate and acetate excretion increased. The presence of oxygen reduced acetate excretion by up to 55% and under the 95% O2-5% CO2 gas phase succinate excretion was reduced 48%. Irrespective of gas phase or the presence of 5-HT, adenine nucleotide pool sizes and the adenylate energy charge remained constant. The addition of 5-HT had no significant effect in increasing the specific activity of phosphofructokinase (PFK). In the presence of ATP the addition of cAMP increased PFK activity by up to 48%; AMP enhanced the enzyme activity by up to 69%, irrespective of the presence or absence of 5-HT. These results are discussed in terms of the effects of 5-HT, gas phase, and nucleotide pools on the motility, metabolism, and control of H. diminuta, and of other parasites which respond to 5-HT.