Phosphoenolpyruvate carboxykinase activity increases in the kidney after glucocorticoid administration and in acidosis. In both cases, a selective stimulation of the synthesis of phosphoenolpyurvate carboxykinase can account for the augmentation of the enzyme level. Using an assay based on the translation of phosphoenolpyruvate carboxykinase mRNA in a heterologous cell-free protein synthesizing system, we show that the glucocorticoids and acidosis, acting by independent mechanisms, cause an increase in the level of functional mRNA coding for the enzyme of sufficient magnitude to explain the increase in the rate of enzyme synthesis.