Fecal estrogen concentrations were monitored during pregnancy and the immediate postpartum period by enzyme immunoassay (EIA) in four zoo animal species. The gorillas, sable antelopes and tapir showed a steady increase in estrogen concentrations throughout pregnancy, with maximum values recorded just before parturition and rapid decline to basal values immediately after parturition. In contrast, estrogen concentrations in the Grevy's zebra peaked by mid-gestation and declined during the final stages of gestation. The determination of fecal estrogen concentrations by EIA provides a practical method for diagnosis and confirmation of pregnancy in many zoo animals but must be used cautiously, particularly if extended to species whose profiles are not clearly established.