To assess the effects on the biosynthesis of collagen types I and III associated with an acute increase in blood pressure, we established a mid-thoracic aortic coarctation in the rabbit and studied gene expression and protein accumulation of these collagen types proximal to the stenosis 1, 3 and 7 days and 2, 4 and 8 weeks after coarctation. The mRNA level of type I collagen pro-α2(I) was maximal at 3 days and returned to normal at 4 weeks. mRNA of pro-α2(I) was localized mainly in the outer media, adventitia and intima. Accumulation of type I collagen and its precursors was increased by 3 days, peaked at 4 weeks, and decreased toward normal by 8 weeks, corresponding to the distribution of pro-α2(I) mRNA. Gene expression for pro-α1(III) was similar to that of pro-α2(I) but was distributed throughout the media. We conclude that the mechanical stresses associated with an acutely induced alteration in pressure initiate rapid gene expression for collagen types I and III in the aortic wall. The response for collagen type I, predominantly in the outer media and adventitia, suggests that these regions play an immediate role in the resistance to excessive dilatation of the aorta. The diffuse response for collagen type III in the media suggests participation in a more extensive remodeling response associated with the reinforcement and reorganization of the musculo-elastic fascicles.