The first human xenotransplants were made in 1920 in France, by a professor of Russian origin, Serge Voronoff (1866-1951). At the age of 18, he left Russia to study medicine in Paris; he became a naturalized Frenchman in 1895. Dr. Alexis Carrel taught his young friend, an ingenious and skillful surgeon, the technique of transplanting. In 1896, Voronoff left for Egypt, where he stayed until 1910. There, he took an active interest in eunuchs who, castrated when they were children, revealed certain deficiencies. Voronoff was convinced according to his own observations that testicles not only have a genital function, but also that they act on the skeletal, muscular, nervous, and psychological development of the individual. Already in June 1889, physiologist Adolphe Brown-Séquard (1817-1894) injected himself under the skin with an aqueous extract of dog and guinea pig testicles, ground up and mollified–an opotherapy or juice treatment. In line with the eugenicist trend in medicine of the 1920’s and 1930’s, Voronoff intended to “rejuvenate” human organisms with a transplant of glands from chimpanzees and baboons, who were thus elevated to the rank of brotherly species with mankind. “I dare assert,” he wrote, “that the monkey is superior to man by the sturdiness of its body, the quality of its organs, and the absence of those defects, hereditary and acquired, with which the main part of mankind is afflicted.” For him, aging was the result of a slowing down of endocrinal secretions, and particularly sexual hormones. Brown-Séquard’s experiments soon proved inefficient. But Voronoff had already transplanted chimpanzee thyroids on people suffering from thyroidal anomalies. And a transplant of a chimpanzee’s bone on a wounded soldier in 1915 suggested to him the idea of transplanting a monkey’s testicle in a man. According to him, glandular transplants would allow the production of the hormone for an extended time period, contrary to opotherapy which required repeated injections with not really convincing results. Between 1917 and 1926 Voronoff tested out his theory on animals, doing more than 500 homo-transplants on rams, goats, and even a bull. According to his observations, older animals transplanted with younger animals’ testicles regained lost vigor.