Vaginal physiology was evaluated in 23 postmenopausal women before estrogen replacement therapy and at 1, 3, 6, 12, 18, and 24 months while receiving conjugated equine estrogens (Premarin). Reversal of hormonal levels (17β-estradiol, gonadotropins) and vaginal cytology occurred within one month. Vaginal pH levels significantly decreased from a baseline mean of 5.2 to a level of 4.2 at 24 months (P<.05). Women who were sexually active showed a greater decline in pH levels than did women who were sexually inactive. Maximum increases in amount of vaginal fluid and potassium levels were observed after three months of therapy. Vaginal blood flow and vaginal electropotential difference were significantly increased over baseline values at one month and again at 12 months (P<.05) with a slow progressive improvement continuing throughout 24 months of estrogen replacement therapy. This study provides documented laboratory evidence to suggest that restoration of vaginal tissue function requires 18 to 24 months and explains why dyspareunia may persist in the early months of replacement therapy despite hormonal and cytologic return to premenopausal values.