Changes in 14C content of metabolites in photosynthesising Chlorella have been investigated following the addition of iodoacetic acid or ammonium chloride. The 14C content of phosphoglyeric acid increases in the former and decreases in the latter case. Both reagents divert 14C from sucrose and polysaccharide into compounds of and associated with the tricarboxylic acid cycle. The principal site of iodoacetic acid inhibition is triose phosphate dehydrogenase whereas ammonium chloride stimulates the conversion of phosphoglyceric acid to phosphoenolpyruvate and pyruciv acid. The latter suggests a direct effect of the ammonium ion on phosphoglyceromutase, enolase, or pyruvic kinase in vivo. Changes in the concentration of alanine, aspartic acid, glutamic acid, serine, and glutamine upon addition of the ammonium ion are not well correlated with their 14C labelling patterns.