The effect of L‐thyroxine on the metabolism of collagen in rats was studied by analyzing the urine for free and total hydroxyproline. In addition the hydroxyproline of the neutral salt and citrate soluble fractions of the skin was determined. Both free and total hydroxyproline were equally increased in the urine after the administration of L‐thyroxine. The increase was of the same magnitude in two, four and twelve months old rats, although the basal excretion values of hydroxyproline were significantly lower in the older rats. The content of hydroxyproline in both neutral salt and citrate soluble fractions of the skin was slightly decreased after the administration of L‐thyroxine.