Interstitial collagen types I and III are the predominant collagens in the amniotic and chorionic connective tissues. However, this matrix also contains proteoglycans, fibronectin, laminin, and elastin, which together with the collagens may undergo partial degradation prior to fetal membrane rupture at term. In this study, stromelysin (MMP-3) and tissue inhibitor of metalloproteinases-1 (TIMP-1) were immunolocalized in fetal membranes obtained at term prior to labor. MMP-3 stained the cells of the amniotic epithelium, fibroblasts and macrophages of the amniotic and chorionic matrix, and those of the chorionic cytotrophoblast; there was no staining in the maternal decidua. TIMP-1 showed a similar staining pattern, except that the staining was darker in some amniotic epithelial cells and was present in the maternal decidua. The maternal decidua produces the two human relaxins H1 and H2; the latter, when incubated with explants of human fetal membranes, caused a dose-dependent and significant increase in expression of the MMP-3 gene and its secreted protein into the media. A significant effect of relaxin H2 on 92-kDa gelatinase (MMP-9) gene expression was also shown--an effect requiring poly(A)+ RNA rather than total RNA. Both relaxin H1 and H2 caused a significant increase in secretion of MMP-9 protein and its enzyme activity in the media. The magnitude of the effects of the two relaxins was similar, in contrast to findings from other biological studies in which relaxin H2 was shown to be more active. Neither of the relaxins had any effect on 72-kDa gelatinase (MMP-2) activity or on the TIMP-1 protein or its activity. This study suggests that local relaxins may be involved in the degradation of the complex fetal membrane extracellular matrix and may cause activation of an enzyme cascade resulting in fully activated MMP-9. Such effects could be important in the degradative pathways occurring in the amnion and chorion in the peripartal period.