The testes are important organs of the male reproductive system with the dual function of spermatogenesis and steroidogenesis. Testosterone (T), the principal androgen, maintains reproductive organ function and stimulates sperm production in adult males. In most spaceflight studies, reduced testes weight and T levels were reported (Table 1). Reduction in T following spaceflight is not always accompanied by reduced spermatogenesis. Reduction in T levels may not be great enough or the duration may not be long enough to affect spermatogenesis.
Systematic examination of androgen-dependent accessory sex glands may provide valuable insight. However, to our knowledge no researchers have systematically examined the effects of spaceflight on the accessory sex glands. In one published study, no effects on T were reported. Inconsistent results may occur because T levels in the blood fluctuate across the circadian cycle. Ortiz et al., avoiding the bias inherent in single measurement studies by collecting 24-h urinary T data, reported increased postflight urinary T in rats followed by a return to normal T levels.