Incubation of L-929 cells in growth medium containing excess NaCl induces cellular protein synthesis to become resistant to the inhibitory action of growth medium made hypertonic by the addition of Na-acetate, KCl, choline chloride, D-alanine, or sucrose. Hypertonic preincubation also induces resistance to the inhibitory action of the drugs dimethyl sulfoxide, ethanol, and L-1-tosylamido-2-phenylethyl chloromethyl ketone, but not to the action of NaF or the inhibitors of polypeptide chain elongation, emetine and cycloheximide. Induction is independent of the solute used to make the induction medium hypertonic and is not mediated by an elevated intracellular Na+ concentration. In a separate series of experiments, it was determined that the formation of functional methionyl-tRNAf.40 S ribosomal complexes is not impaired by exposure of uninduced cells to hypertonic growth medium. The combined results are evaluated with respect to the mechanism of action of hypertonic growth medium in the inhibition of polypeptide chain initiation and the adaptive mechanisms that animal cells have evolved to deal with alterations in their extracellular environments.