In previous work, we demonstrated in animals and humans an antidopaminergic effect of estradiol at the level of the striatum. In the present study, we tested the effect of a large dose of estradiol (0.5 mg s.c.) administered either acutely or during several days in four female ovariectomized monkeys, displaying a persistent buccolingual dyskinesia due primarily to a midbrain lesion, but which is markedly enhanced by dopaminergic agonists. One of the monkeys also displayed a lesion-induced parkinsonian-like tremor of the opposite limbs. Chronic administration of estradiol markedly reduced the apomorphine-induced potentiation of the dyskinesia but did not affect the tremor. A single dose of estradiol was followed after 24 h by a 75% reduction of the effect of apomorphine on the dyskinesia but a 50% increase in the response to apomorphine was seen after 2 weeks. The response was at the control level after 30 days. Domperidone, a peripheral dopamine agonist that does not cross the blood-brain barrier and which causes an elevation of prolactin similar to that seen after estradiol, is followed by a similar biphasic modification of the response to apomorphine. Our results suggest that estradiol may have opposite effects on the sensitivity of the striatal dopamine receptors and therefore on dyskinesia, depending on the time of observation. An elevation of prolactin appears to have similar effects. Moreover, some effects of these hormones may be delayed by several days to weeks in primates.