AbstractParamecium tetraurelia, stock 51s, will not survive if shifted from normal medium to medium with 80–100 mM NaCl. But, if first kept in medium with 50 mM NaCl for at least 45–60 min, cells survive in medium with 80–100 mM NaCl. After shifting to medium with 50 mM NaCl, cells increase internal levels of ninhydrin‐positive substances (NPS) for the first 30 min, then decrease them again over the next 90 min. Total internal osmolarity probably remains constant over this 90 min since contractile vacuole output is constant. Treatment with the protein synthesis inhibitor G418 (geneticin) prevents acclimation in cells in 50 mM NaCl, but does not prevent the changes in NPS levels. Thus NPS are not sufficient to account for acclimation, nor is there evidence that they are r