In the present study, we examine the roles of 1) explicit information about language provided to learners prior to treatment and 2) aptitude (specifically grammatical sensitivity) within Processing Instruction. Forty-two learners of Spanish in their third-semester of study were divided into two groups: those who received explicit information (EI) prior to treatment (+EI) and those who did not (-EI). All participants also took the grammatical sensitivity portion of the Modern Language Aptitude Test. Treatment consisted of 50 structured input activities based on VanPatten and Cadierno (1993), in which learners heard a sentence and then indicated what they thought they heard by selecting one of two drawings. The processing problem for the treatment was the First Noun Principle and the target structure was clitic direct object pronouns with object-verb-subject (OVS) and subject-object-verb (SOV) word orders. Treatment was delivered via computer using SuperLab 4.0. Participants' responses were tracked and the measurement taken was trials-to-criterion: how long it took participants to begin processing sentences correctly. Results using ANOVA procedure revealed that EI did not make a difference: the +EI and -EI groups began processing sentences correctly at about the same point during the treatment. Results using standard "r" correlations also yielded no correlation between grammatical sensitivity and trials-to-criterion. (Contains 5 notes and 3 tables.)