It is widely accepted that some male rats fail to copulate because of a decrease in arousability, measured as decreased general locomotor activity (hypoactivity). This relationship, however, failed to explain an observation made in our laboratory that rats that failed to copulate exhibited increased general locomotor behavior. To directly address this issue, we quantified open-field and male sexual behaviors in 360 rats from two different strains. Twenty-two out of 49 hyposexual males were also hyperactive; this was a significantly greater number than would be expected by chance (p < 0.002, binomial test). Interestingly, only 6 of the 49 hyposexual males were hypoactive; this number was actually significantly smaller than would be expected by chance (p < 0.02). There was no correlation between behavioral measures and plasma levels of testosterone or progesterone. A decrease in selective attention and a failure to be stimulated by amphetamine was apparent in all hyperactive rats--those normal-sexual as well as hyposexual. The hyperactive rats were not hypertensive. We conclude that a significant percentage of hyposexual rats are hyperactive, and that hypoactive rats generally exhibit normal levels of sexual behavior. Decreased selective attention and decreased responsiveness to amphetamine do not explain this result, which is also not related to blood pressure or androgen levels.