The total intracellular amino acid profiles of Giardia intestinalis trophozoites, Trichomonas vaginalis, and Crithidia luciliae were determined by sensitive amino acid analysis. The three protozoan parasites exhibited distinctively different amino acid profiles, but all three were dominated by high concentrations of intracellular alanine. This common feature suggests that alanine synthesis is a major aspect of intermediary metabolism in these protozoan parasites. There were also distinctively different aspects, particularly those related to arginine metabolism. Ornithine, citrulline, and ammonia were found in G. intestinalis trophozoites, but no intracellular arginine was detected. This pattern is consistent with the high activity of giardial arginine deiminase and the arginine dihydrolase pathway. However, in contrast, both T. vaginalis and C. luciliae contained considerable intracellular pools of arginine. When the G. intestinalis trophozoites were divided into the two populations existing in in vitro culture--attached and nonattached--there were no significant differences between the amino acid profiles of the two populations, with the exception of citrulline, which was found in lower concentrations in the nonattached cells. The T. vaginalis profile was characterised by high concentrations of valine and leucine, whereas the C. luciliae profile was dominated by high levels of glutamate and proline. Overall, the analysis of the total amino acid pool provides a valuable technique to rapidly highlight those amino acids of potential metabolic significance and to provide a rapid technique for defining the nature of amino acid metabolic interactions in situ.