Implantation of Crayfish testes in sexual rest into females leads to a new onset of spermatogenetic activity characterized by a precocious appearance of spermatocytes. However, these cells degenerate: their evolution never goes further than the pachytene stage of meiosis prophase. Abortive prophases appear continuously; this also takes place in acini which develop secondarily. Implantation of testes in sexual rest into males in the same physiological shape also leads to the precocious appearance of primary spermatocytes blocked in prophase of meiosis. However, the transplant tends to reacquire a structure similar to the one of a normal resting testis. The delay of appearance of the first spermatocytes is always shortened--less than a month instead of one month and a half--if the receptive female is destalked at the time of the implantation or if the donor male has been destalked one month before the transplantation. The interpretation of the observed phenomena is discussed.