It has been well documented that virgin female rats previously systemically sensitized against H-Y antigen almost invariably reject male skin isografts of adult origin. However, when the donor is a neonatal animal, these H-Y-incompatible grafts are often permanently accepted. Furthermore, such neonatal male skin grafts are frequently able to induce a state of unresponsiveness to subsequent grafts of adult male skin. This ability to induce tolerance is evidently dependent upon the persistence of the neonatal skin graft as it does not occur if the neonatal graft is rejected. Thus, the behavior of H-Y-incompatible neonatal skin grafts in rats parallels their behavior in mice.