AbstractA practical method for the quantitative measurement of the estrogenic steroid estradiol‐17β in the feces of pigtailed macaques (Macaca nemestrina) is described. The method, which includes homogenization, filtration, ether extraction, and sephadex purification, produces an 85.3% recovery of 3H‐estradiol. Comparable recovery of 3H‐progesterone (64.1%) and 3H‐testoster‐one (67.9%) is also obtained. Estradiol, measured by radioimmunoassay methodology, has inter‐ and intra‐assay variations of 7.1% and 4.3%, respectively. Linearity of estradiol was checked by varying the amount of feces analyzed. Results show a correlation of 0.9945, Y = 32.526x + 0.205. Accuracy of estradiol, determined by adding radioinert estradiol to the feces before homogenization, was 98.7% with a correlation of 0.9950, Y = 1.027x − 0.543. The menstrual cycles of several animals were analyzed and corresponded well with serum levels of estradiol‐17β. Similar correspondence was obtained in matched fecal and serum samples from pregnant animals. These methods provide a practical and accurate solution to the problem of collecting hormone data in field studies without the potential complications of capture in laboratory research.