The mechanisms that regulate the expression of genes encoding extracellular matrix proteins in fibroblasts and other mesenchymal cells have remained elusive. Studies from several laboratories have indicated that Tax, a trans-regulatory protein from the human T cell leukemia virus type I not only augments viral gene expression but also triggers the expression of various cellular genes. Here, we examined the hypothesis that the expression of collagen genes may also be modulated by Tax. NIH-3T3 cells were simultaneously transfected with a Tax expressor plasmid and a chimeric construct containing regulatory sequences (-804 to +42 bp) of the alpha 1(I) procollagen gene (COL1A1) promoter. The results indicated that the promoter activity of the -804 to bp COL1A1 fragment increased up to 12-fold in cells expressing Tax. Deletion analysis revealed that the region of COL1A1 encompassing nucleotides -174 to -84 contained the Tax-responsive elements. A gene segment encompassing nucleotides -187 to -67, which contained this region, proved sufficient to confer Tax inducibility (2.5-fold) to a herpes simplex virus thymidine kinase promoter. Stably transfected NIH-3T3 cell clones that constitutively produce Tax displayed elevated levels of alpha 1(I) procollagen and fibronectin transcripts and increased production and accelerated processing of type I procollagen. These findings suggest that retroviral proteins may be involved in the pathogenesis of idiopathic diseases accompanied by collagen overproduction.