Spatial abilities vary during the menstrual cycle. The effects of a similar rhythm in men are unknown. Mental rotation and spatial visualization of young healthy volunteers (29 females and 31 males) were tested during the menstrual and periovulatory phase of the menstrual cycle in women, and during the low-testosterone and high-testosterone phases of the circatrigintan rhythm in men in random order. Men outperformed women in both tests. In women, higher scores were achieved during the periovulatory phase, in men during the low-testosterone phase. The correlation between actual salivary testosterone levels and spatial abilities were positive in women and negative in men. According to our knowledge, this is the first study showing effects of a male infradian rhythm on cognitive abilities. (Contains 3 figures.)