Previous research has documented correlations between endogenous testosterone levels and visuospatial cognitive function. Some causal relations have also been established in treatment designs in which testosterone was administered to elderly subjects for a number of weeks. Particularly, one study reported a selective effect of a single administration of testosterone on some aspects of spatial memory in 15 women. The present study tested the hypothesis whether a single administration of 0.5 mg of sublingual testosterone would improve visuospatial ability in healthy young women on a test that has consistently been associated with male superiority. Twenty-six women participated in a double-blind placebo-controlled cross-over trial of single administration of testosterone and placebo. Subjects were tested in the same phase of the menstrual-cycle. Four to five hours after both administrations, subjects completed a standardized measure of visuospatial ability (3-D Mental Rotations Test). Visuospatial ability improved significantly after testosterone administration compared to placebo, after controlling for learning effects due to repeated testing. Testosterone is suggested to be causally related to visuospatial ability in young women.