Growing surface cultures of 3T3 cells possess a low level of glycerophosphate dehydrogenase, an important enzyme in triglyceride synthesis. When 3T3-C2, a subline that does not undergo appreciable adipose conversion, reaches confluence, the level of the enzyme does not increase. In 3T3-F442A, a subline that undergoes the conversion with high frequency, the specific activity of the enzyme increases about 600-fold. The enzyme of the adipose 3T3 cells is different from that of non-adipose 3T3 cells in its thermal stability and its affinity for dihydroxyacetone phosphate. The enzyme of the adipose cells probably corresponds to the stable “adult” form of the enzyme, as described previously, and the enzyme of non-adipose 3T3 cells is probably the unstable “embryonic” form. For this reason, the change in the enzyme that takes place during the adipose conversion is greater than would be indicated simply by the total increase in specific activity. If, as seems likely, the two forms of glycerophosphate dehydrogenase are the products of independent genes, the adipose conversion may activate a hitherto silent gene for the stable enzyme.