An increase in the biosynthetic rate of the brain-type isozyme of creatine kinase (CKBB, first described in the uterus as the "estrogen-induced protein") was found in the ovary, vagina and estrogen receptor-rich regions of the brain (preoptic area, anterior hypothalamus and median eminence), one hour after injection of 5 micrograms of estradiol-17 beta into 25-28 day-old rats. The increase in synthetic rate in the ovary, detected by 35S methionine incorporation, peaked at 1h, but still remained higher than in control ovaries at 6 h and was reflected in a longer-term increase in ovarian CK specific activity after 4 daily injections. Both ovary and vagina, similarly to brain, contained exclusively the BB isozyme of CK. These findings suggest that the entire female reproductive system can respond to estrogen by a rapid induction of CKBB.