Metoclopramide is an antidopaminergic drug that binds D2 receptor at the central nervous system and peripheral levels. Recent studies have demonstrated the presence of a sexually dimorphic nucleus in the anterior hypothalamus of rats and humans. The sexual differentiation of brain and its expression on sexual behavior is directed by catecholamines and steroids, which in turn will determine the masculinization of the preoptic area of the hypothalamus. Metoclopramide blocking dopaminergic receptor inhibits dopamine action and therefore brain masculinization. To determine the effects of the administration of this antidopaminergic drug in neonatal male rats, the design of the study considered measurements of mounts, penis intromission, and ejaculation in adult male rats treated neonatally with 100 μg metoclopramide. The administration of the drug or the vehicle (control) was by subcutaneous route during the third and fifth days of postnatal life. Six of 10 neonatally treated rats did not express male sexual activity in adulthood, while all control rats showed the characteristic pattern of male sexual behavior. Frequency of ejaculation in the rats treated with metoclopramide was 1.5 ± 1 as compared to 5.0 ± 0.9 in control rats (p < .01). There were meaningful differences with respect to the frequency of intromission in experimental rats (16 ± 8) compared to control rats (115 ± 18; p < .01). There is strong evidence that metoclopramide neonatally administered inhibits male sexual behavior in adult rats. Dopamine may be implicated in the process of neonatal brain sexual differentiation.