The effects of Crinofizin (Cfz), cortisone, triiodothyronine (T3) and methylthiouracil (MTU), alone or in association with one another, on collagen metabolism of impuberal and adult rabbits were tested. These effects were assessed by the hydroxyprolinuria (Hyp) assay corroborated with observations regarding mortality, body weight, diuresis and excretion of nitrogenous substances. Cfz given alone stimulated weight gain and did not alter characteristically the Hyp. Cortisone and T3 given in various doses and ways without Cfz induced general phenomena of the antianabolic or catabolic type, affecting characteristically the Hyp, which was low after administration of cortisone and increased after T3; association of Cfz to both cortisone and T3 had general protecting and corrective effects and brought Hyp towards or at normal values. Chronic administration of MTU with or without Cfz did not have significant effects. The corrective value of Cfz by anabolic and anti-catabolic actions on collagen metabolism disturbed by cortisone or T3 in excess is discussed.