Abstract
SUMMARY Artificial cryptorchidism is followed about the 21st day after operation by a transient increase in the numbers of interstitial and Leydig cells of the testis of the rat. At other times the total numbers of these cells are not significantly altered, but the proportion of non-senile Leydig cells is increased over the whole experimental period. The 21st day is the period of lowest production of androgen (as measured by citric acid and fructose levels in the accessory reproductive organs). The transient rise in cell numbers is considered to be due to the operation of the normal pituitary-gonad relationship, and the subsequent fall is attributed to the diminished ability of the heat-damaged interstitial tissue to react to gonadotrophins.