Hypoxic modulation of collagen metabolism appears to be related to pathogenesis of many diseases such as fibrosis of connective tissue after injury and scleroderma. Since most of our understanding of how procollagen assembles within the cell has come from studies on cells cultured under normoxia, it may not be helpful for the etiology of the diseases observed in peripheral tissues under hypoxic conditions. As an experimental model for the hypoxic modulation of collagen metabolism, we cultured 3T3-L1 fibroblasts under low partial oxygen pressure and found that hypoxia enhances secretion of type IV collagen 10-fold and accelerates adipose conversion of the cells. The enhanced secretion of type IV collagen was not accompanied by an appreciable increase of alpha1(IV) and alpha2(IV) mRNAs. Prolyl 4-hydroxylase alpha increased only 3-fold under hypoxia. We suggest that hypoxia creates an environment of prolyl 4-hydroxylase alpha(2)beta(2) tetramers favorable for the folding of type IV procollagen which has many interruptions of the Gly-Xaa-Yaa repeat.