The low Km system for sulfate uptake in Lemma is dramatically down-regulated by prior growth in cystine or high concentrations of sulfate. To look for possible site(s) of regulation, measurement was made of the steady-state concentrations of S-containing compounds in Lemna grown in concentrations of sulfate or cystine which down-regulate the uptake system to about the same extent. The results indicate that down-regulation is mediated by inorganic sulfate (or a product of its metabolism) in the case of growth in sulfate, but by cyst(e)ine (or a product of its metabolism) in the case of growth in cystine. In neither case is the product methionine. The apparent down-regulation of the low Km system by about 40% by growth in methionine was shown to be a nonspecific effect. Properties of Lemna uptake systems for neutral amino acids, basic amino acids, purines, choline, ethanolamine, aldohexoses, and urea were determined. Measurement of kinetic constants and cross-competition experiments indicate that each is a separate system and that each has high affinity for the transported molecule. Structural specificity of the systems showed a range from broad specificity (i.e., any neutral L-Alpha-amino acid; and aldohexose), to very strict structural requirements (i.e., choline; ethanolamine; urea).