In healthy humans given IV amphetamine, males have a larger amphetamine induced release of dopamine in the ventral striatum and an increased subjective experience of high. However, when 3 doses of oral amphetamine are given to healthy subjects, females show a sensitization in self-ratings of vigor. Thus, although the males have a larger initial response, females may have more sensitization. The sex differences are manifest after just 3 exposures to stimulants. This suggests that females may be more vulnerable to developing an increased responsiveness to stimulants after just 2 prior doses.