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Urinary excretion of hydroxyproline, considered as an index of collagen metabolism, was determined prior to, and during, a period of fracture healing in normal and hypothyroid rats. Hypothyroidism was induced by feeding the animals for eight months with a diet containing thiouracil (0.3%). A closed complete fracture of the humerus was produced and left unsupported for 28 days. At the end of this healing period, hydroxyproline of fractured and intact humerus was also determined. Significant increase of a saline extractable and total hydroxyproline was found in fractured bone of the normal rat and this was considered due to the synthesis of new collagen. No evidence of such a change in saline extractable hydroxyproline was found in hypothyroid rats. Significant progressive decrease of urinary hydroxyproline was found in hypothyroid rats during the period of fracture healing. Under the conditions of this experiment, evidence of deficient collagen synthesis and metabolism in hypothyroid rats is confirmed.