The effects of 4 weeks of calcium supplementation on free-and total testosterone levels were established in active and sedentary adult males at rest and exhaustion. Thirty healthy male athletes were equally divided into three study groups, as follows: Group 1-non-exercising subjects receiving 35 mg calcium/kg body weight; Group 2-subjects receiving 35 mg calcium/kg body weight undergoing training routines for 90 min/day, 5 days a week and Group 3-subjects undergoing training routines for 90 min/day, 5 days a week. The testosterone levels were determined before and after supplementation, at rest and following a hard training routine. The plasma free-and total testosterone levels increased at exhaustion before and after supplementation relative to resting values (p<0.05). This was also true when active subjects were compared to inactive subjects (p<0.05). Our results show that training results in increased testosterone levels in athletes and that the increase is greater if accompanied by calcium supplementation, which may be useful for increasing overall athletic performance. © Humana Press Inc. 2008.