It is still unclear whether sex differences in cognitive functioning are mainly due to perinatal organizing effects of sex hormones on the brain, or to activating effects in adulthood. In a group of 22 female-to-male transsexuals a battery of visuospatial and verbal ability tests was administered twice: shortly before and 3 months after the start of androgen treatment. The administering of androgens was clearly associated with an increase in spatial ability performance. In contrast, it had a deteriorating effect on verbal fluency tasks. This study offers preliminary evidence that androgens directly and quickly affect cognitive performance in females.