Uteri of pregnant rats on Days 6, 7 and 8 of pregnancy were studied to determine the histochemical distribution of collagen types III and V and the incorporation of [3H]glycine into fibrillar collagens during the period of embryonic implantation. Types III and V had a similar distribution in the non-decidual stromal region and muscle layers in implantation sites. They were found to have very low levels in the primary decidual tissue on Day 6 and were not detected in developing decidual tissues on Days 7 or 8. Following injection of labelled glycine, collagen was extracted and the specific activity of the collagens determined by fluorography and 3H incorporated into the collagen bands in the gels. It was found that incorporation of label into both types I and III was similar (33.4+/-12.0 and 31.8+/-18.1 cpm microg-1 collagen respectively) but 3.5 times that of type V (7.7+/-5.3 cpm microg-1). These studies suggest that although fibrillar collagens are metabolized or redistributed in the growing decidual tissue, they are incorporated rapidly into the extracellular matrix during remodelling of the outer stroma and muscle tissues.