The elephant-in-mousehole doctrine holds that "Congress ... does not alter the fundamental details of a regulatory scheme in vague terms or ancillary provisions--it does not, one might say, hide elephants in mouseholes." Whitman v. American Trucking Association, 531 U.S. 457, 468 (2001). This doctrine is an exception to the textualism approach to constitutional interpretation.
American Trucking's elephant-in-mousehole doctrine was first implied by FDA v. Brown & Williamson Tobacco Corp., 529 U.S. 120 (2000), where the Court held that nicotine was not a "drug" within the meaning of the statute (the FDCA) and thus could not be regulated by the FDA. Although a pure textualism approach could find that nicotine fit within the FDCA's technical definition of a "drug", the Court held that "we are confident that Congress could not have intended to delegate a decision of such economic and political significance to an agency in so cryptic a fashion." Brown & Williamson, 529 U.S. at 160.
The Court also applied the elephant-in-mousehole doctrine in Gonzales v. Oregon, 546 U.S. 243 (2006). It held that the Attorney General lacks authority under the physician-registration provision of the Controlled Substances Act ("CSA") to prohibit physicians from prescribing drugs for use in physician-assisted suicide. The Court found implausible "[t]he idea that Congress gave the Attorney General such broad and unusual authority through an implicit delegation in the CSA's registration provision." Gonzales, 546 U.S. at 267 (citing American Trucking and Brown & Williamson).
Courts of Appeals have been applying the elephant-in-mousehole doctrine. Compare Am. Bar Ass'n v. F.T.C., 368 U.S. App. D.C. 368, 430 F.3d 457 (D.C. Cir. 2005) (finding elephant-in-mousehole where Federal Trade Commission claimed authority under financial consumer privacy statute to regulate attorneys) with Am. Fed'n of Gov't Employees, AFL-CIO v. Gates, 486 F.3d 1316 (D.C. Cir. 2007) (finding no elephant-in-mousehole where the Department of Defense claimed authority under National Defense Authorization Act to curtail collective bargaining with civilian employees) (2008); NISH v. Rumsfeld, 348 F.3d 1263, 1269 (10th Cir. 2003) ("We simply do not see the elephant in the mousehole" where the military claimed statutory authority to give blind vendors priority in awarding mess hall contracts).