Xenobiotic compounds introduced into the environment by human activity have been shown to adversely affect the endocrine system of wildlife. Various species exhibit abnormalities of (1) plasma sex steroid hormones, (2) altered steroid synthesis form the gonad in vitro and (3) altered steroidogenic enzyme function. These endpoints are sensitive and relatively easy to measure quantitatively with reliability and precision. These observations have led to the conclusion that sex steroid hormones could be markers of exposure to, and altered function from, endocrine disrupting contaminants (EDCs). However, there are serious limitations in the use of steroid hormones as generalized markers of EDC exposure. Steroid hormones exhibit seasonal, ontogenetic, gender and species-specific variation. Moreover, the regulation of sex steroid plasma concentrations is a relatively complex phenomenon capable of shorterm (minutes — hours) alteration due to environmental inputs, such as acute stress -- an activational response. Alterations in steroid synthesis and degradation also can be a response to altered embryonic development due to EDC exposure — an organizational response. If steroid hormones are to be used as biomarkers, then closely controlled, well designed sampling has to be performed. Additionally, an appreciation of the variation possible in endocrine responses among the species to be studied must be obtained.