Abstract
The advantages of testicular sperm head counts are many, and the procedure should be encouraged. It is simple, quantitative, objective, and has excellent sensitivity. Effects can be measured for all stages of spermatogenesis, and indirect hormonal effects on spermatogenesis also can be detected. A large range for a dose-response curve exists, and there is a direct relationship to fertility in the animal. It is the only parameter that is really suitable for interspecies extrapolation, and in humans it is the parameter that can be applied to the quantitative assessment of reproductive risk.