Arginine is rapidly depleted from the medium during the cultivation of T. vaginalis in a defined or semi-defined medium. It is broken down to ornithine, ammonia and carbon dioxide by the three enzymes of the dihydrolase pathway: arginine deiminase, catabolic ornithine carbamyltransferase (OCTase) and carbamate kinase. Arginase and urease as well as citrulline hydrolase appear to be absent. Ornithine, a product of the pathway was further converted to putrescine by an active ornithine decarboxylase. Apparent substrate Km values determined were arginine deiminase, 103 microM; catabolic OCTase, 71 microM; ornithine decarboxylase 134 microM. A substrate level phosphorylation is associated with the pathway; the significance of this to the overall energy economy of the cell is unclear.