A chronic noncancer toxicity assessment for 1,3-butadiene (BD) has been conducted by the Texas Commission on Environmental Quality (TCEQ) using information not available to the U.S. Environmental Protection Agency (U.S. EPA) in 2002. The TCEQ developed a chronic reference value (ReV) of 33 microg/m3 (15 ppb). The chronic ReV is based on the same animal study and critical endpoint used by U.S. EPA for ovarian atrophy in B6C3F1 mice, but uses mode of action (MOA) information that indicates the diepoxide metabolite is responsible for ovarian atrophy. In addition, diepoxide-specific hemoglobin adduct data in mice, rats, and humans and other experimental data that became available after 2002 were used to support a conservative data-derived toxicokinetic animal-to-human uncertainty factor (UFA) of 0.3. The default toxicodynamic UFA of 3 was used, together with the data-derived toxicokinetic UFA of 0.3, resulting in a total UFA of 1. The necessary experimental data were not available to calculate a chemical-specific adjustment factor, although supporting data suggest the toxicokinetic UFA may range from 0.01 to 0.2. The chronic ReV value, along with a unit risk factor developed by the TCEQ, will be used to evaluate ambient air monitoring data so that the general public is protected against adverse health effects from chronic exposure to BD.