The activity, protein concentration and catalytic efficiency of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase was determined in rats aged 1 to 199 days. Microsomal enzyme total activity peaked on day 24, during weaning, and again on day 63, during the onset of puberty. Increased enzyme activity during weaning resulted primarily from an increase in the catalytic efficiency of the enzyme with a slight reduction in enzyme protein content. The rise in enzyme activity during the onset of puberty, however, was primarily the result of an increase in enzyme protein concentration. Thus, the activity of reductase in mammalian livers reflects, at different stages in development, the modulating influence of both the total number of reductase molecules and the catalytic efficiency of the enzyme.