Abstract. The aim of this study was to evaluate the estrogenic activity of tuberous samples of phytoestrogen-rich Pueraria mirifica collected from 25 of 76 provinces in Thailand by vaginal cornification assay. Tuberous powders were prepared and administered to ovariectomized rats for 14 consecutive days at dosages of 10, 100 and 1,000 mg/kg BW respectively, and were compared with a daily treatment with 2 mg/kg BW 17β-estradiol (E2). Rats treated with 10 mg/kg BW Pueraria mirifica showed no vaginal cornification. Treatment with 100 mg/kg BW Pueraria mirifica from 13 out of 25 plant samples resulted in development of vaginal cornification. The cell count percentages of the vaginal smeared cells for the treatment with the 2 plant samples that exhibited the fastest vaginal cornification revealed large variation in their estrogenic activities. Treatment with 1,000 mg/kg BW Pueraria mirifica from all plant samples produced vaginal cornification with the mean value for the period (day) of first appearance of cornified cells being 4.08 days compared to 2 days with 2 mg/kg BW E2. The overall appearance period (day) of cornified cells during the treatment and post-treatment period with 1,000 mg/kg BW per day Pueraria mirifica was shorter than treatment with 2 mg/kg BW E2. The results demonstrate that the plant population shows differential estrogenic activity as evaluated by vaginal cornification assay. , and isoflavones [5][6][7][8][9][10]. The estrogenic activity of miroestrol was first determined in ovariectomized rats. Administration of miroestrol produces a mammogenic effect in rats [11]. Using the vaginal cornification assay, miroestrol was estimated to have 0.25 times the estrogenic activity of 17β-estradiol [12]. Thus, various types of research have been conducted on the estrogenic effects of P. mirifica, especially in recent years. Crude P. mirifica powder can relief the climacteric symptoms of post-menopausal women [13]. The estrogenic activities of P. mirifica powder and extract has been evaluated in various species of experimental animals and found to have no toxicity [14]. P. mirifica has biphasic effects on MCF-7 cell proliferation, both suppression and stimulation [15], and an anti-proliferation effect on HeLa cells (ERα-negative human cervical adenocarcinoma cells) [16]. Considering the effects on the reproductive organs and reproductive-related hormones, P. mirifica suppresses luteinizing