The fetal membranes undergo striking changes in structure before delivery that involve catabolism of the extracellular matrix. To investigate the role of specific enzymes in this process, we examined gelatinase activities in rat amnion, visceral yolk sac placenta, and placenta and amniotic fluid between Days 18-21 of pregnancy. Matrix metalloproteinase (MMP)-2 was present in amnion on all days, and its activity increased slightly on Day 21. The 92-kDa gelatinase, MMP-9, was not detected on Days 18-20 but appeared by the morning of Day 21. There was a marked increase in MMP-9 mRNA in the amnion on Day 20, preceding the appearance of MMP-9 activity. Western blotting confirmed an increase in MMP-9 protein in amnion on Day 21. MMP-2 and MMP-9 activities were detected in extracts of whole yolk sac placenta, placenta, and amniotic fluid, but there were no striking changes in these gelatinases between Days 18 and 21. However, the capsular regions of the visceral yolk sac placentae, which thin and rupture during labor, did show higher MMP-9 activity on Day 21 than on Days 18 and 20. We suggest that the striking increase in MMP-9 expression in amnion and possibly the capsular region of the visceral yolk sac placenta approximately 12 h prior to delivery is responsible, in part, for the alterations in the structure of these fetal membranes before parturition.