Over the last 50 years, reproductive health has declined in both males and females of human populations. Evidence includes increased prevalence of genital abnormalities in boys (hypospadia and cryptorchidism), accelerated sexual maturation in girls, increased prevalence of testicular and breast cancers, and poor semen quality. During this period, developmental and reproductive anomalies have also been documented in wildlife species, and environmental chemicals with endocrine-mimetic properties are suspected as likely causative agents. Epidemiologic and field evidence suggests that the developmental and reproductive pathologies observed above are most prevalent in cultivated areas and are correlated with pesticide use. About 80% of pesticide application is herbicidal, and currently atrazine is one of the most common herbicidal contaminants of ground and surface waters. Furthermore, atrazine is purported to be an endocrine disruptor. Amphibians serve as early sentinels of environmental toxicants due to their common occurrence in and sensitivity to pesticide-containing waters. Reports in the literature indicate strong adverse effects in frogs due to environmental exposures to atrazine, including demasculinization and/or feminization of male gonads, reduced hematocrit, hypophagia, diminished survival, reduced growth, and smaller laryngeal muscles. The effects on gonadal development occur in response to exposures ranging from 0.1 to 25 μg/L, including concentrations below the standard of 3 μg/L established by the US EPA for drinking water. By comparison, environmental exposures for humans can be as high as 50 μg/L atrazine. Atrazine's adverse effects on amphibians at low doses have received little attention in the laboratory despite evidence of altered gonadal structures in the presence of low atrazine levels. The purpose of this project is to gain more information about the low dose effects of atrazine on amphibians. Experiments will involve native species of amphibians obtained from local populations, an important extension of previous work involving atrazine in model organisms. Larval amphibians will be exposed to one of six environmentally-relevant doses of atrazine through the entire larval period. Behavioral and developmental anomalies will be assessed at the organismal, tissue, and biochemical levels of biological organization. Emphasis will be placed on gonadal and endocrine development. The results of these studies will provide insights into the effects of low-dose exposure to atrazine on non-target species. Regulatory agencies may use this information in decisions regarding the registration and use of atrazine. There is a critical need to develop animal models to indicate the impact of agrochemicals, including endocrine-disrupting compounds, on non-target species. The development of native aquatic larval amphibian models into research and regulatory tools for assessing the impact of an agrochemical on non-target species will be very valuable in protecting wildlife. These models will also be useful to the agrochemical industry in evaluating the toxicity of existing and proposed products.