The effect of phenylbutazone in daily doses of 100 mg/kg on collagen metabolism in rats was studied after an injection of 14C-proline, by determining specificand total 14C-hydroxyproline activity in the skin collagen fractions and urine. It was found that a 21-day administration of phenylbutazone influenced the earlyphases of collagen metabolism by inhibiting collagen biosynthesis. The later phases of collagen turnover (maturation) were not affected by phenylbutazone treatment. In the given dose, phenylbutazone did not stimulate the catabolism of either newly-formed, soluble collagen, or mature, insoluble collagen.