The uptake and accumulation of methionine and other amino acids in the cestode H. diminuta is studied with respect to the influence of pyridoxal, pyridoxine, and ammonium ion. It is found that neither pyridoxal nor pyridoxine exert any measurable effect upon the initial velocity or the total amount of uptake of methionine. Concentrations of pyridoxal and pyridoxine ranged from .005 mM to 1.0 mM, and concentrations of methionine ranged from .05 mM to 1.0 mM. Ammonium ion is found to exert a very significant effect on the total accumulation of methionine and several other amino acids but no effect on the initial velocity of uptake. The ammonium ion affects accumulation in two ways: First it increases production of a particular metabolite within the worm which is composed partly of methionine and certain other amino acids. Production of this metabolite in turn decreases the amounts of constituent amino acids in the worm' s amino acid pool, thereby allowing for an increased apparent accumulation of those amino acids. The second way the ammonium ion affect accumulation is by inhibiting efflux of some of the neutral amino acids, particularly methionine and alanine. It can exert is inhibitory effect when present only in the efflux medium or only in the worm and not in the efflux medium, or when present both within and without the worm. An attempt to explain this unusual effect is made by postulating a new transport mechanism.