Ascorbic acid stimulates collagen gene expression in cultured fibroblasts but mechanisms responsible for this effect are poorly understood. In the presence of the transitional metal iron, ascorbic acid could induce lipid peroxidation with the formation of reactive aldehydes. Because another aldehyde, acetaldehyde, the first metabolite of ethanol, also stimulates collagen transcription in cultured fibroblasts, we investigated whether ascorbic acid induces lipid peroxidation in cultured cells and if this is the mechanism by which ascorbic acid stimulates collagen gene expression. Ascorbic acid (0.2 mmol/L) induced lipid peroxidation and stimulated collagen alpha 1(I) gene transcription in cultured human fibroblasts. Inhibition of the ascorbic acid-induced lipid peroxidation in cultured human fibroblasts with alpha-tocopherol (50 mumol/L) or methylene blue (10 mumol/L) prevented the stimulation of collagen gene expression. Addition of malondialdehyde (200 mumol/L), a product of lipid peroxidation, to cultured human fibroblasts also increased two- to threefold collagen production and procollagen alpha 1(I) mRNA levels. Thus, ascorbic acid induces lipid peroxidation and reactive aldehydes, and this step may be necessary for stimulation of collagen gene expression by ascorbic acid in cultured human fibroblasts.