The Nation's Report Card[TM] informs the public about the academic achievement of elementary and secondary students in the United States and its jurisdictions, including Puerto Rico. In 2003, a trial NAEP mathematics assessment was administered in Spanish to public school students at grades 4 and 8 in Puerto Rico. Based on preliminary analyses of the 2003 data, changes were made in administration and translation procedures for the 2005 NAEP administration in Puerto Rico. This report describes the content and administration of the trial NAEP mathematics assessments in Puerto Rico in 2003 and 2005, problems with item misfit in the 2003 data, results of a special validity analysis, and plans to integrate Puerto Rico into the national sample in future administrations. (Contains 16 tables and 6 figures.)