The effect of arginine on the growth and metabolism of Giardia intestinalis trophozoites was determined. Supplementation of the normal growth medium (Diamond's TYI-S-33) with 5 or 10 mM arginine accelerated trophozoite growth over the first 2 days. There was a corresponding rapid utilisation of arginine, with none being detectable after this time. The decrease was associated with the appearance in the growth medium of 1 mol of ornithine and 2 mol of ammonia per mol of arginine utilised, the stoichiometry being consistent with the operation of the arginine dihydrolase pathway. Subsequently, there was a decrease in the ammonia concentration in the medium. Removal of arginine from the medium by pretreatment with arginase substantially decreased cell growth. In TYI-S-33 medium containing no added glucose, instead of the normal 50 mM glucose concentration, arginine supplementation also increased cell growth over the first 2 days, with concurrent stoichiometric production of ornithine and ammonia. However, in these conditions, the ammonia concentration remained elevated. This suggests that under normal conditions there is re-uptake of ammonia, which is glucose dependent. The observations confirm the operation of a functional arginine dihydrolase pathway in G. intestinalis. The concordance of cessation of rapid growth with the depletion of arginine, and the beneficial effect on growth of arginine supplementation suggests that arginine availability is a limiting factor during the initial stages of rapid growth. It would appear that arginine is a major potential energy source during the initial stages of giardial growth, and that supplementation of Diamond's TYI-S-33 medium with additional arginine may provide an improved in vitro culture medium.