EPA published a proposed test rule under section 4 of the Toxics Substances and Control Act (TSCA) in 1992 covering a number of aryl phosphate base stocks. On March 30, 1993, EPA announced initiation of negotiations with the Aryl Phosphates Panel of the Chemical Manufacturers Association (now the American Chemistry Council or ACC) to develop a TSCA section 4 Enforceable Consent Agreement (ECA) for aryl phosphate base stocks as an alternative approach to testing under the proposed rule (58 FR 16669). On October 9, 1998, EPA sent letters to the Chief Executive Officers of companies, including those who were participating in the development of this ECA, to announce EPA's High Production Volume (HPV) Challenge Program. Consistent with the International Organization for Economic Co-Operation and Development (OECD) Screening Information Data Set (SIDS) Program, EPA's HPV Challenge Program encourages US chemical producers and importers to voluntarily provide existing screening level data, or, if none exist, to develop such data on US HPV chemicals. Because some overlap of testing in the HPV Challenge and this ECA initiative were identified, the industry committed to develop the screening level data for the HPV Challenge Program before continuing with further development of the ECA. In this way, results from the HPV Challenge program would feed back into consideration of needs for the ECA testing and, where possible, could avert some or all of the potential overlap testing. EPA is evaluating the need for any additional testing of the subject aryl phosphate base stocks under an ECA or rulemaking.