It is well known that female rats developing close to a male in utero show a higher frequency of heterotypical or male-like behavior in adulthood, and have longer anogenital distances. The present investigation was designed to evaluate the in utero masculine influence on the homotypical sexual behavior of male and female rats. Also explored was the influence on body weight on Gestation Day 21 (day of cesarean delivery) and 21, 40. and 120 days after birth, testicle weight following the gonadectomy of males on Day 40, and serum testosterone in males and females on the day of delivery. The presence of a contiguous male fetus located caudally with respect to uterine blood flow led to the masculinization of male-like behavior in male rats, the defeminization of female like behavior in females and to increased body weights on Day 21 of gestation in both sexes. No significant differences were detected in the remaining parameters. Findings indicated a gradation in the intensity of expression of male and female sexual behavior in adulthood related to the intrauterine position resulting in interindividual variability. The possible implication of this physiological phenomenon in the structure of rodent populations is discussed.