The parabiotic union of two animals, rats in particular, is an established technique1P2 which was used in several hundreds of studies.3 Until recently, however, this technique was rarely used for purposes of homoplasty, particularly in mammals. Meanwhile, it is natural to suppose that by placing two organisms in common circulation, tissue incompatibility can be reduced and the host rendered tolerant to the donor. In 1936, Schwind4-6 reported successful homotransplantation of an extra tail or leg in litter mate rats placed in temporary parabiosis so that the transplanted organ was transferred from the one organism to the other by the use of a pedicle graft technique, widely employed clinically in surgical autoplasty. In 1940, Lapchin~ky,’-~ using a similar technique, replaced amputated legs in rats, ranging in age from 15 to 25 days, with a leg taken from another litter mate placed in temporary parabiosis to the host for a period of up to 21 days (FIGURES I , 2, and 3). The matured rats used the transplanted “other’s” leg for more than two years (until their natural death) (FIGURE 4 ) . It should be noted that, as a rule, rats of the same litter are rarely monozygous, a quality which makes possible successful homotransplantation of organs (kidneys) even in human patients.1° Lapchinsky8 in his experiments achieved survival of homografted extremities using a combination of 2 a male donor and a female recipient. This fact shows that the experimental rats were nonmonozygous. The role age played in parabiosis in such experiments became of major significance, due to the subsequent findings of Hasek,” who showed the induction of the immunological tolerance of the host to the donor by embryonic parabiosis in birds. Joining the circulation of two developing eggs through the windows made in the shells, Hasek found that in the hens matured from the parabiotic eggs, the homografted skin survived in the same way as in autografting. This phenomenon was termed “tolerance” by Billingham, Brent, and Medawar,lg who simultaneously with Hasek, showed in similar experiments on mice that the intrauterine administration of spleen tissue and bone marrow taken from the prospective donor reduces incompatibility in adult mice subjected to skin transplantation from the said donor. Further experiments by Puza” showed that tolerance can be induced not only during the period of embryonic development, but also within the first postnatal days, the “adaptive” period varying in different animals. Now, the question arises whether or not the rats used by Schwind and Lapchinsky in the parabiotic experiments were in the adaptive period when tolerance can be induced as a result of blood exchange, and whether i t is possible to achieve similar results using parabiosis in adult rats. That the question is pertinent is also shown by the fact that Shipachev,14 using a temporary parabiosis, failed in homoplasty of fingers from a leg of one adult patient to a hand of the other. To find the answer, we have, since October 1961, carried out experiments on 54 pairs of adult rats, undoubtedly beyond the adaptive period, which were placed
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