The effect of hydralazine on several parameters of collagen biosynthesis has been studied in cultured human skin fibroblasts. Cells treated with hydralazine synthesized procollagen which was severely deficient in hydroxyproline and hydroxylysine, indicating inhibition of prolyl and lysyl hydroxylase reactions in the cell. Assays of prolyl and lysyl hydroxylase activities, however, revealed markedly increased levels in hydralazine-treated cells. The stimulatory effect of hydralazine could not be simulated in cell extracts, demonstrating its requirement for intact cells. The effect occurred slowly over a period of 96 h and was dependent on hydralazine concentration between 10 and 100 microM. This phenomenon was also observed in lysyl hydroxylase-deficient mutants. In both normal and mutant cells the relative magnitude of the hydralazine effect could be modified by ascorbic acid in the culture medium. Ascorbic acid increased the response of prolyl hydroxylase to hydralazine from 1.5- to 2-fold to 3- to 7-fold, whereas it decreased the response of lysyl hydroxylase to hydralazine from 4- to 8-fold to 2- to 3-fold. Total collagen synthesis was substantially reduced in hydralazine-treated cells; the time course and the dose-response relationship were similar to those observed for the hydroxylases. alpha, alpha'-Dipyridyl, an iron chelator, mimicked these effects of hydralazine. The studies suggest the existence in cultured cells of a compensatory mechanism for overproduction of these crucial enzymes in collagen biosynthesis, a mechanism which remains functional in cells derived from patients afflicted with hydroxylysine-deficient collagen disease.