Optimum temperature and temperature coefficient of protein synthesis in young wheat plants exhibit phenotypical temperature adaptation. In plants grown for 2 days at either chilling (4 C), medium (20 C), or high (36 C) temperature the respective values are: 27 C and 14.2 kilocalories per mole, 31 C and 18.2 kilocalories per mole, 35 C and 23.6 kilocalories per mole, based on in vivo [(14)C]leucine incorporation into total protein. The validity of the [(14)C]leucine incubation method has been confirmed by double-labeling experiments. Readaptation time curves are complex: the optimum temperature parameter readjusts within approximately 4 hours to an altered temperature regime, whereas the temperature coefficient needs between 4 and 96 hours for complete readaptation-depending on the temperature conditions prior to the temperature shift. Heat-preadapted plants need a recovery period at medium temperature to regain their cold adaptability with respect to optimum temperature. Cycloheximide (30 micrograms per milliliter) reduces [(14)C]leucine incorporation into protein by 85%, thus indicating that predominantly the cytoplasmic 80S system of protein synthesis is involved in temperature adaptation.