In 1974, rumors spread that the Mexican government and foreign doctors were colluding to sterilize schoolchildren. The allegations sparked panic across the Valley of Mexico and prompted popular protests and school closures. Public officials and mainstream journalists portrayed worried parents as ignorant pawns in a conservative conspiracy. Ordinary residents and leftist groups, meanwhile, accused the government of using family planning to uproot opposition or even to eliminate the poor. This article analyzes the epistemological contests that surfaced in response to the rumors. Contrary to the claims of cultural and political elites, hearsay was not opposed to informed engagement but rather an integral component of it. As literacy, readerships, and political consciousness increased, so too did the efforts to understand and influence the news by talking about it. At the center of Mexico City's protracted political opening were debates over what counted as legitimate knowledge production.