Fragments of spleen autotransplanted to subcutaneous tissue of the abdomen in the rat undergo almost complete necrosis and then regenerate into splenic tissue with a microscopic structure indistinguishable from the structure of the original organ. The regenerative process reminiscent of the spleen’s embryogenesis, originates from a shell of surviving splenic tissue at the surface of the implant. The regenerative zone first consists of almost monotonous connective tissue cells interspersed with red blood cells; it develops into splenic red pulp consisting of sinuses and intersinal cords. As capillaries develop, the structure of small arteries and peri-arterial lymphatic sheaths appear, and soon the structure of splenic white pulp becomes evident. Some 5 wk after autotransplantation, the splenic reconstruction is complete. The weight of the recovered tissue is a linear function of the weight of the implanted tissue; yet the linearity is lost when the weight of the implanted tissue exceeds 100 mg.