Congress enacted the Horse Protection Act (Act), 15 U.S.C. §§ 1821-31, in an effort to end the practice of soring, which involved the use of performance-training devices that injured show horses. Secretary of Agriculture Richard Lyng (Secretary) (defendant) promulgated implementing regulations that imposed civil and criminal penalties for the use of specific soring devices and, more broadly, any devices that caused or could reasonably be expected to cause soring. The notice of proposed rulemaking acknowledged that the results from ongoing studies might indicate a need to prohibit additional specific soring devices. After a major university completed a study that identified a number of devices not prohibited by existing regulations, the American Horse Protection Association, Inc. (Association) (plaintiff) petitioned the Secretary to amend the regulations to prohibit the use of the specific devices identified by the study. The Secretary denied the petition. The Association filed suit in federal district court, challenging the Secretary’s refusal to undertake new rulemaking to revise the soring regulations. The district court granted summary judgment to the Secretary. The Association appealed.