Primary, 7,12-dimethylbenz[a]anthracene-induced, and transplanted MTW9 mammary tumors of the rat were used as models to study regression in hormone-dependent tumors. Three observations are reported: 1) The in vivo utilization of oxygen and glucosewas equally large in regressing and in growing tumors; 2) incorporation of labeled 14C-glucose or 5-3H-uridine and/or 2-14C-glycine into DNA, protein, RNA, and lipids of regressing tumors remained surprisingly high, although lower than during growth; and 3) RNA and protein-specific activity of prelebeled tumors decreased sharply in both nuclear and cytoplasmic Fractions during the first days of regression. Decrease in specific activities was similar in regressing and in growing tumors. These results are interpreted to indicate that synthesis of cell constituents occurs at the onset of tumor regression.