It is also well known that the adult brain has tremendous neural plasticity, and manipulation of hormones results in significant improvements in various cognitive domains (6). In the study by Dr. Hoff et al., estrogen and progesterone levels were measured on a weekly basis for 4 weeks. The resulting data were then averaged. One can wonder whether these results were statistically significant, given the large variations during women’s menstrual cycles; the range of estradiol has been reported as 50–600 pmol (Canada) or 40–260 pg/ml (U.S.) (7). More important, however, is the fact that not all estrogens are equal. Comparing endogenous estrogen with conjugated equine estrogen is not scientifically valid, since the human radioimmunoassay for 17β-estradiol does not measure levels of estrone sulfate, equilin sulfate, 17α-dihydroequilin sulfate, equilin, equilenin, 17β-dihydroequilin, or 17β-dihydroequilenin, which are all metabolic products of conjugated equine estrogen (7).