‘Voronoff’s hypothesis was this: hormones, like testosterone produced by the testes, would reverse aging by a process he called “rejuvenation.” One of his first experiments used himself as a test subject. He injected ground up dog and guinea pig testicles under his own skin, but was disappointed when this did not result in any verifiable effect. He reasoned that living grafts of testicular tissue, rather than injections, would have a more dramatic and lasting rejuvenation effect.
This lead to cross-species glandular transplantation surgeries. His early experiments involved transplanting thyroid tissue into humans with a thyroid deficiency. He also began transplanting the testicles of executed criminals into rich old guys (as a treatment for senility and schizophrenia), but had to stop when the demand for the procedure far exceeding the supply of criminal testicles. At this point, Voronoff began using monkey testicles instead, and his first “monkey gland” to human transplant took place in June of 1920.’