In this study, we examine explicit information and aptitude within processing instruction. Forty-six learners of German in their third semester of study were divided into two groups: those who received explicit information prior to treatment (+EI) and those who did not (-EI). Participants also took the grammatical sensitivity portion of the Modern Language Aptitude Test. Treatment consisted of structured input activities in which learners heard a sentence and indicated comprehension by selecting between two drawings. The processing problem was the First-Noun Principle, and the target structure was nominative-accusative case marking on masculine nouns in object-verb-subject and subject-verb-object sentences. Treatment was delivered via computer (SuperLab 4.0). The measurement taken was trials to criterion: how long it took participants to begin processing sentences correctly. Results revealed that the +EI group began processing sentences correctly before the -EI group. As for aptitude, grammatical sensitivity correlated weakly with the scores in the +EI group, but not in the -EI group. (Contains 3 tables and 3 notes.)