The photosynthetic incorporation of H14CO3- by the alga Platymonas subcordiformis decreased with increasing concentrations of NaCl in the medium. There was no correlation between the response of P. subcordiformis to salinity in high or low NaCl-concentrations in the growing solution. When other osmotic substances were used (KCl, sucrose, sorbitol, manitol, xylitol) the incorporation of 14C also decreased. Under low salt conditions the decline of photosynthesis depended on the NaCl concentration of the culture medium (adaptation).In contrast to photosynthesis incorporation of H14CO3- in the dark increased with hypo- or hyperosmotic stress. The increase depended on the concentration of added NaCl (or other osmotic substances) and on the salinity of the growing solution (adaptation).The incorporation of radiocarbon into the neutral fraction (carbohydrates) during photosynthesis was remarkably stimulated by hyperosmotic conditions whereas hypoosmotic stress caused a decrease. This was due to the change of the content of mannitol under different osmotic stresses.Compared with the control incorporation rates of 14C into amino acids and organic acids were lower with both hypo- and hyperosmotic stress. As a consequence of the declining incorporation of 14C into mannitol after hypoosmotic stress the distribution of radiocarbon in the amino acid, organic acid, and neutral fraction was changed. Incorporation rates of 14C into the control and into algae after hyperosmotic stress were in the order, neutral fraction ≫ amino acids > organic acids, while incorporation rates after hypoosmotic stress were in the order, amino acids > organic acids, neutral fraction. The most labelled individual amino acids were glutamate, alanine, aspartate, and proline. The incorporation into alanine increased with hyperosmotic conditions. The most labelled organic acids were citrate and malate. 14C-fixation in the dark resulted in strong labelling of amino acids and organic acids and low incorporation into the neutral fraction. Fixation rates of 14C with hyperosmotic stress were in the order, amino acids ≫ neutral fraction > organic acids. In constrast, the rates with hypoosmotic conditions were in the order, organic acids > amino acids ≫ neutral fraction. The major portion of the radioactivity of amino acids was found in glutamate, proline, and aspartate. With hyperosmotic stress the incorporation of 14C into these amino acids increased, while after hypoosmotic stress the incorporation declined. With hyperosmotic conditions a remarkable increase of the organic acids, especially that of malate, was observed. From these data it is suggested that Platymonas has different metabolic responses to hyper- and hyposmotic stress, although with both osmotic stresses the same general effects were observed: a decline in the rate of photosynthesis and stimulation of CO2-fixation in the dark.