In roosters, fertility peaks to 96% at 32 weeks, shortly after sexual maturation, and then declines rapidly to 68% at 70 weeks and to less than 10% at 110 weeks, as a result of intratesticular retention of spermatozoa. The reduction in fertility is associated with functional structural changes of the interstitial tissue, reflected in decreased plasma androgen levels from 2.7 ng/ml at 32 weeks to less than 0.5 ng/ml at 110 weeks. In high fertility roosters, the interstitial tissue is tightly packed with Leydig cells, which contain relatively large amounts of rough endoplasmic reticulum and lipid droplets, both related to androgen synthesis. In the old rooster, which has a low fertility, the interstitial tissue contains only occasional Leydig cells within an enlarged intercellular space. These Leydig cells contain small amounts of endoplasmic reticulum, mainly rough, and there are low plasma androgen levels. It is concluded that differentiation of roosters' interstitial tissue is reflected by plasma levels of androgen. This, in turn, is related to the mechanism of spermatozoa release from Sertoli cells and, consequently, with the level of fertility.