Serum inhibin, FSH, and testosterone levels were measured by radioimmunoassay in 36 normal human males, aged 8 to 63 years. Inhibin and FSH levels rose during 8 to 17 years to reach adult values. At these ages there is a significant direct linear correlation between serum FSH and inhibin levels (r= 0.81, p > 0.01), whereas in adulthood the relationship between serum FSH and inhibin levels was significant inverse (r = -0.70, p < 0.01). No correlation was found between serum testosterone and FSH (r = - 0.18, p:NS) or between serum testosterone and inhibin (r = 0.14, p:NS). In senescence there is a significant decrease in the serum inhibin levels (r = -0.53,p < 0.05). These findings suggest that negative feedback is established once serum inhibin has reached adult values, and that in physiologic conditions inhibin plays a greater role than testosterone in the negative feedback regulation of FSH. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.