The correlation between fecal and serum levels of 17ß estradiol (E) during the estrous cycle, follicular phase (F1 ) luteal phase (F2 ) and early pregnancy (F3-F6 ) in mares was determined by radioimmuno assay (RIA) with the objective of developing a noninvasive method for monitoring equine reproduction. Ten Arabian mares from 4 to 1 O years of age were divided into two groups determined by nursing status: G 1 with sucking foal, n=4 and G2 non-nursing, n=6. Ali were bred during their second heat with 6 resulting pregnancies. Blood and fecal samples were taken during the follicular phase (Fl ), luteal phase (F2}, at 0-1 O days (F3), 12 to 16 days (F4 ), 18 to 36 days (F5) and 50 days (F6) of pregnancy. Hormone levels in blood (10 ml) and fecal samples (50 g) were determined using Diagnostic Products Corporatíon (DPC) RIA test kits. In the fecal samples, the hormone was previously extracted by the Wasser technique. Hormone leve Is were 52.522 ± 18.240 in the serum and 460.797 ± 95.311 in the feces, compared to 59.558 ± 23.489 (serum) and 338.431±69.737 (feces) duringgestation. The correlation coefficient, r=0.3411 (n=500), was statistically significant (P<0.05), reflecting a relationship between serum and fecal hormone levels and indicating that detection of 17ß estradiol E2 levels in the feces is a viable noninvasive technique for determining reproductive status in mares.