Rat pregnancy is characterized by a progressive continuous induction of apoptosis in the maternal tissues lining the conceptus. Different cell types (decidual cells, giant cells, epithelial cells, etc.) undergo apoptosis at the different stages of development. We demonstrated here that, independently from their origin and stage of pregnancy, the dying cells were invariably characterized by the presence of 'tissue' transglutaminase (tTG) and negligible amounts of Bcl-2 proteins. This finding confirms the antithetic roles played by these two gene products within the apoptotic pathway. It is noteworthy that Bcl-2 was detected in decidual cells just after the implant; however, starting from day 9, its positive detection was reduced in the maternal tissues and appeared in the embryonal ones. The apoptotic nature of death was also suggested by the presence of a high phagocytic activity in the maternal involuting regions and by morphological observations showing that the tTG positive cell remnants displayed typical phenotypic features of apoptosis. The possibility that the cells lining the conceptus that die by apoptosis could be envisaged as an inert barrier between the maternal and fetal tissues is discussed.