In June 1979, the Environmental Protection Agency (EPA) promulgated new source performance standards (NSPS) to control particulate and sulfur dioxide emissions from coal-fired power plants. In Sierra Club v. Costle,¹ decided in April 1981, the District of Columbia Circuit Court of Appeals upheld the standards against an array of substantive and procedural challenges by both industry and environmental groups. The decision is notable not only because of the substantial impacts of the challenged standards on national coal use, energy costs, and the utility industry, but also because of the great respect shown by the court for EPA's substantive standard-setting authority. There may be even greater implications, however, in the court's holding that communications between EPA and outside interests after the close of the comment period for the rulemaking were permissible under the Clean Air Act's² prescribed rulemaking proceedings. This could prove to be a landmark development in administrative law that opens the door considerably to ex parte contacts in all informal rulemaking proceedings.