We investigated a non-invasive method of specimen collection for determining the changes of reproductive hormones in aged menopausal monkeys after a long-term feeding of the Thai herb Pueraria mirifica (PM) containing phytoestrogens. Three groups of aged menopausal monkeys (n = three in each group) were fed daily with 10, 100, or 1000 mg of PM for a 90 day treatment period, and fed with distilled water for 30 and 60 days of the pre- and post-treatment periods, respectively. Urine samples were collected for 14 h daily every 5 days and assayed for follicle stimulating hormone (FSH), luteinizing hormone (LH) and estradiol levels. The result showed that monkeys fed with PM 10, PM 100, and PM 1000 had a decrease in urinary FSH levels during the treatment period, followed by a rebound increase during the post-treatment period. Urinary estradiol levels tended to decrease and fluctuated between 4.28 and 266.71, 2.85–42.27, and 6.24–203.50% of the pre-treatment levels in those three groups, respectively. Decreases in urinary LH levels could not be observed in all the three groups. These results suggest that FSH could be a candidate marker to detect the estrogenic effects of phytoestrogens in aged menopausal monkeys when changes of urinary hormones need to be used as an indicator.