This study explores whether fetal allograft survival is age dependent and organ specific. Fetal rat tissue (renal, gonadal, hepatic) from the third trimester of gestation (days 15-21) was transplanted into 306 outbred adult rats for 10-30 days. Grafts were studied by morphometric and histologic analysis. Ten days after implantation, renal tissue (N = 75) from late gestation (days 19-21) showed no increase in size. In contrast, 17-day fetal grafts (N = 20) grew 6.8 +/- 3.4 times,* while 15-day fetal grafts (N = 28) grew 17.5 +/- 6.1* times. (The symbol "*" indicates p less than 0.05, compared to original size). Twenty days after implantation, these 15-day fetal grafts (N = 20) grew 48.8 +/- 17.7* times. Ten days after grafting, the younger fetal tissue showed excellent maturation of renal elements and no sign of rejection; older fetal grafts had poor renal architecture and a dense lymphocytic infiltrate. The 15-day fetal gonadal tissue (N = 18) showed a moderate 10.6 +/- 3.2* increase in size while the 15-day hepatic grafts (N = 16) were regularly rejected within 10 days. Selected fetal allografts from early in the third trimester can not only survive but can grow and mature in an immunocompetent recipient. This fetal graft growth appears to be both age dependent and organ specific. The use of fetal organs may broaden the potential pool for transplantation. However, further studies are needed to define the ontogeny of graft acceptance.