SUMMARY

Plasma testosterone concentration and the weights of the seminal vesicles and ventral prostate gland were measured in normal and cryptorchid Fischer rats at 3, 4\m=.\5,7\m=.\5and 13\m=.\5months of age, and in normal parabionts and cryptorchid parabionts of 13\m=.\5months of age. Testosterone was measured individually by a protein-binding method. In normal rats, all parameters rose to a maximum at 7\m=.\5months of age, then levelled off or declined at 13\m=.\5months of age. In cryptorchid rats, a similar pattern at a lower level was found for accessory sex gland weight, but plasma testosterone levels showed a progressive decline from an above normal level at 3 months to a subnormal level at 13\m=.\5months of age. Cryptorchid parabionts were less responsive to gonadotrophin stimulation from union with a castrated partner than normal parabionts. The present study showed that plasma testosterone levels in normal and cryptorchid rats are age-related. It also showed that the pattern of plasma testosterone levels observed in cryptorchid rats is different from that seen in normal rats. Accessory sex gland weight is also age-related but is not a reliable index of plasma testosterone levels