Chlorpyrifos was selected for EPA's Endocrine Disruptor Screening Program (EDSP) based on widespread use and potential for human and environmental exposures. The purpose of the program is to screen chemicals for their potential to interact with the estrogen, androgen, or thyroid pathways. A battery of 11 assays was completed for chlorpyrifos in accordance with test guidelines developed for EDSP Tier 1 screening. To determine potential endocrine activity, a weight-of-evidence (WoE) evaluation was completed for chlorpyrifos, which included the integration of EDSP assay results with data from regulatory guideline studies and the published literature. This WoE approach was based on the OECD conceptual framework for testing and assessment of potential endocrine-disrupting chemicals and consisted of a systematic evaluation of data, progressing from simple to complex across multiple levels of biological organization. The conclusion of the WoE evaluation is that chlorpyrifos demonstrates no potential to interact with the estrogen, androgen, or thyroid pathways at doses below the dose levels that inhibit cholinesterase. Therefore, regulatory exposure limits for chlorpyrifos, which are based on cholinesterase inhibition, are sufficient to protect against potential endocrine alterations. Based on the results of this WoE evaluation, there is no scientific justification for pursuing additional endocrine testing for chlorpyrifos.