In assessing the scope of Congress' authority under the Commerce Clause, we stress that the task before us is a modest one. We need not determine whether respondents' activities, taken in the aggregate, substantially affect interstate commerce in fact, but only whether a "rational basis" exists for so concluding. Given the enforcement difficulties that attend distinguishing between marijuana cultivated locally and marijuana grown elsewhere, 21 U.S.C. § 801(5), and concerns about diversion into illicit channels, we have no difficulty concluding that Congress had a rational basis for believing that failure to regulate the intrastate manufacture and possession of marijuana would leave a gaping hole in the CSA.