The present studies assessed the extent to which adult sex differences in responsiveness to both acute and repeated amphetamine (AMPH) treatment can be attributed to differential exposure to testosterone (T) during the early critical period for sexual differentiation. At birth, male pups were sham-operated or gonadectomized, whereas female pups were given T or an oil injection. In adulthood, all animals were gonadectomized or sham-operated. Locomotor activity in response to either 1.5 mg/kg AMPH (IP) or the saline vehicle was measured for 2 h every third day, on five occasions. On the sixth occasion, all animals received 0.75 mg/kg AMPH (IP) in a test for sensitization. In Experiment 1, animals were tested in the absence of circulating gonadal hormones, whereas in Experiment 2, all animals received 5.0 micrograms estradiol benzoate (SC), 30-35 min prior to each behavioral test. Results indicate that neonatal exposure to T suppresses responsiveness to AMPH in adulthood. The differences between neonatal T-exposure groups were magnified in the presence of circulating estradiol. The fact that female animals were more responsive to AMPH regardless of neonatal T exposure suggests that lifetime exposure to estradiol alters responsiveness to this hormone, and to AMPH, in adult animals and/or that exposure to T both pre- and postnatally is necessary for the full suppression of responsiveness seen in adult male animals.