Various fractions of hydroxyproline were determined in skin, femora, and humeri of DBA/2 mice treated for 2 weeks with one of three hormones: bovine growth hormone, anabolic steroid (17-β-hydroxy-17-α-methylandrostano[3,2-c]pyrazole), and cortisone acetate. The following fractions were studied: 0.15 M NaCl, 0.50 M NaCl, 0.50 M citrate, and total hydroxyproline. Since collagen is the only protein in the body containing hydroxyproline in significant amounts, this fractionation method allows an appraisal of the rates of synthesis, maturation, and breakdown of collagen. Growth hormone produced a significant increase in all extractable fractions and in the total hydroxyproline content of tissues studied. The action of anabolic steroid on hydroxyproline was generally comparable to that of growth hormone, but was less marked. Both total hydroxyproline and saline-extractable fractions were significantly reduced in skin and bone of cortisone-treated mice.