The steady state concentrations of S-containing compounds formed in Lemna paucicostata Hegelm. 6746 in response to variations in source and concentrations of sulfur were measured. Neither growth rates nor protein accumulation were markedly affected by the various growth conditions. Ignoring complications due to possible compartmentation, the results are consistent with internal pools of both SO(4) (2-) and cyst(e)ine (or products of their metabolism), but not methionine, being effectors of regulation of high affinity SO(4) (2-) uptake. As SO(4) (2-) in the growth medium was increased to 10 mm, down-regulation of high affinity SO(4) (2-) uptake was more than compensated for by unregulated uptake via the "non-saturating" uptake system. Tissue inorganic SO(4) (2-) accumulated but formation of reduced sulfur remained constant. Some conversion of l-cystine sulfur to SO(4) (2-) occurred. Presence of l-cystine in the medium (a) down-regulated high affinity SO(4) (2-) uptake and (b) decreased the rate of SO(4) (2-) organification. The net results were decreased (7 mum l-cystine) or normal (14 mum l-cystine) total tissue SO(4) (2-) and dose-dependent accumulation of soluble cyst(e)ine and glutathione, but not of soluble methionine. l-Methionine was not metabolized to cyst(e)ine or its products. Presence of l-methionine in the medium led to increased total tissue sulfur, accounted for almost wholly by manyfold increases in soluble methionine, AdoMet, and S-methylmethionine sulfonium. Soluble cyst(e)ine increased slightly.