The effect of a combined dose of pregnant mares' serum (PAAS) and human chor ionic gonadotropin (HCG) on male and female rats 2-25 days old was studied. Groups of animals were given injections for three days, then sacrificed on the fourth day. All injections were begun on the 2nd, 7th, 12th, 17th, and 22nd day of life. Ovaries, uteri, seminal vesicles (SV), and ventral prostate (VP) were removed, dissected free of fat, and weighed. Because uterine weight increased earlier than either SV or VP weight, the ability of the uterus to respond to exogenous estradiol and of the SV and VP to respond to exogenous testosterone was examined. The injection schedule, age groups, and day of sacrifice were the same as above. The uterine response to estrogen was found to appear earlier than the VP-SV response totestosterone. The observations suggest that the receptor sites for estrogen may mature earlier than the receptor sites for testosterone.

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