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The differentiation of mouse 3T3-L1 cells is characterized by an accumulation of cytosolic triglyceride and marked increase in many enzymatic activities involved in triglyceride biosynthesis. The specific activity of one such enzyme, pyruvate carboxylase, increases at least 20-fold and is due to a parallel increase in the intracellular concentration of the protein. Pulse-labeling experiments demonstrated that the increase in the specific activity of pyruvate carboxylase was due to an increase in the rate of enzyme synthesis. In the differentiated cell, pyruvate carboxylase represented 1.9% of the total cellular protein and 1% of the protein radiolabeled during a 1-h pulse. This was 35-and 28-fold higher than in the undifferentiated cell, respectively. The turnover of pyruvate carboxylase in the differentiated cell was similar to that in the undifferentiated cell with the enzyme having a half-life of 28-35 h. The half-life of apopyruvate carboxylase in avidin-treated 3T3-L1 cells was 24 h, indicating that the turnover of the apoenzyme was not significantly different than that of the holoenzyme. Radiolabeling pyruvate carboxylase with [14C]biotin and [3H]leucine demonstrated that the turnover of biotin associated with the enzyme was identical to the turnover of the enzymatic protein.