Perchlorate anion (ClO₄^-) is a contaminant found in groundwater and surface waters in locations throughout the United States, including Lake Mead and the Colorado River downstream of Lake Mead. Lake Mead is a large reservoir formed by impoundment of the Colorado River behind Hoover Dam. The reservoir serves as a source of domestic and agricultural water for more than 22 million users and is home to the razorback sucker, an endangered species of fish. Perchlorate enters Lake Mead via contaminated groundwater runoff into the Las Vegas Wash, which enters Lake Mead at Las Vegas Bay. The relatively high concentrations of perchlorate found in the Las Vegas Wash (average 600-800 ppb in 2003) are diluted in the Las Vegas Bay. Results of previous studies indicate disruption of the reproductive systems of common carp (Cyprinus carpio) collected from the Las Vegas Bay of Lake Mead, but a cause has not been identified. Because thyroid and reproductive status and function are related in fish, the current study was designed to determine whether thyroid and reproductive function are altered in carp exposed to perchlorate in Lake Mead. Adult male and female carp were collected in October 2001 from the Las Vegas Bay and from two reference sites in Lake Mead. Endpoints examined included circulating thyroid hormone (thyroxine, or T4, and triiodothyronine, or T3) and sex hormone concentrations, gonadosomatic index (GSI = (gonad weight / body weight) *100)), gonad histology, and thyroid histology. Sex hormones that were measured included the estrogen 17ß-estradiol (E2) and the androgens testosterone (T) and 11-ketotestosterone (11-KT). Data currently available for male fish show no statistically significant effects of the Wash influent on GSI or sex steroids, but thyroid hormone levels appear to be less in male carp caught in the Las Vegas Bay than in those caught at Overton Beach. Mean T4 was less by approximately 2- fold in males caught from Las Vegas Bay than in those caught from Overton Beach. T3 was slightly less in males in more advanced stages of reproductive development caught from Las Vegas Bay than in those caught from Overton Beach. Testis histology suggests that reproductive development might be delayed in male carp captured from Las Vegas Bay, but any effects were not sufficiently severe to affect GSI. The significance of these findings will not be known until histological examinations of thyroid samples from these fish are completed and the findings are put into perspective in relation to activity of the reproductive and thyroid systems. Data currently available for females do not suggest endocrine disruption, but examination of thyroid histology and gonad histology continue. Includes 10 references, extended abstract only.