Male and female rats were weighted daily throughout this study to determine whether neonatal exposure to estrogens influences body weight (BW) patterns, particularly during the period before weaning when vaginal opening (VO) occurs as a consequence of this treatment. Females receiving estradiol benzoate (EB) at the age of 5 days had greater body weight than their controls soon after the treatment and until day 21. Clear 3-day periodic changes of BW, between days 9 and 20, were revealed by the spectral analysis of the results in EB-given females which did not occur in their controls. Changes of BW (at 4-day intervals) were verified in adult control females whereas age-matched EB-treated females did not show such a rhythm. Neither BW gain nor infradian rhythmicity were detected in infantile males after neonatal EB treatment. The results suggest that, as in spontaneous puberty, a relationship between the occurrence of VO and the establishment of the infradian body rhythmicity in the infantile estrogenized female may exist.