The age-related changes of vaginal opening, body weight, the weights of the uterus and ovary, together with histological examination, serum 17β-estradiol (E2) and progesterone levels were examined in intact female Crj:CD ® (SD) IGS rats between 21 and 36 days of age to understand the basic biological profile of changes of the female genital system during sexual maturation in the rat for female pubertal assays. With the beginning of the elevation of serum E2 level from 28 days of age, all

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... ers except body weight started to show drastic change until 31 days of age. The highest incidence of vaginal opening was recorded at 34 days of age. On macroscopic examinations, a number of rats showed uterine imbibition but vaginal opening. Immediately after the confirmation of the vaginal opening, the genital systems of three rats were observed microscopically. Both ovaries already had multiple corpora lutea, and degeneration of endometrial epithelial cells was observed. In conclusion, we obtained essential data on genital tract development of female Crj:CD ® (SD) IGS rats for in vivo screening assays that will contribute to detect potential endocrine active chemicals. In addition, it is assumed that the first ovulation precedes or occurs simultaneously with vaginal opening. KEY WORDS: Crj:CD ® (SD) IGS female rat, female pubertal assay, first ovulation, pubertal onset, vaginal opening.