Testosterone plays a role in the organization of behavior during development. The authors examined whether testosterone could play a maintenance role in behavior as well. In a double-blind manner, verbal and visual memory, spatial cognition, motor speed, cognitive flexibility, and mood in a group of healthy older men who were supplemented for 3 months with testosterone were assessed. The increase in testosterone levels to 150% of baseline levels resulted in a significant enhancement of spatial cognition, but no change in any other cognitive domain was found. Testosterone supplementation influenced the endogenous production of estradiol, and estradiol was found to have an inverse relationship to spatial cognitive performance. These results suggest that testosterone supplementation can modify spatial cognition in older men; however, it is likely that this occurs through testosterone's influence on estrogen.