P4 progesterone levels were determined in fecal samples from two non-pregnant and two pregnant mares at O, 4, 8, 16, 24, 36 and 48 hours post defecation. Subsequently, fecal and serum P4 levels were studied during estrous and the first 50 days of gestation using 4 non-pregnant and 6 pregnant Arab mares. Blood and fecal samples were taken at the same time every other day for 100 days, and progesterone leveis were determined by solíd phase RIA. In the first group of 4 mares, fecal progesterone concentratíon decreased signíficantly (p<0.01) from 26.76 ± 5.24 ng/g at hour O, to 3.7±1.2 ng/g at 48 hours post-excretion. For the second group, the correlation between fecal and serum P4 levels (r=0.96) was significant (p<0.01) during the estrous cycle (r=0.96) and first 50 days of gestation (r=0.79). Overall P4 leve! averages were lower (p<0.05) during the follícular phase (0.38 ± 0.31 ng/ml for serum and 3.72 ± 0.98 ng/g for feces) than during the luteal phase (8.57 ± 4.22 ng/ml for serum and 19.67 ± 8.03 ng/g for feces) of estrous, and significantly (p<0.05) lower during the first 25 days (6.20 ± 2.60 ng/ml for serum and 18.54 ± 4.00 ng/g for feces) than during the subsequent 25 days (8.39 ± 3.86 ng/ml for serum and 21.4 ± 4.74 ng/g for feces) of gestation. These results indícate that ovarian activity and early gestation in the mare can be reliably monitored by measurement of P4 levels in fecal samples.