A major component of reading in subject areas is motivation to engage with text in different and specific ways. Yet, scant research captures fluctuations in adolescents' daily reading motivation in subject areas or across diverse reading activities: important information for supporting discipline-specific reading performance. Thus, this study captures adolescents' fluctuations in reading motivation within and across classroom environments and explores the ways in which teacher practices may bolster student motivation to read in content-area classrooms. This investigation followed 161 students in 6th through 8th grade in three underresourced middle schools in social studies, English language arts, science, and math classrooms (45 classes) with 14 content-area teachers, across multiple days of instruction. Multilevel modeling results show that capturing dynamic understandings of reading motivation--beyond commonly used context-neutral measures--provides unique insight about the flexibility of motivation to read and specifically how environmental factors can influence students' ups and downs in their levels of reading motivation. Findings demonstrate that students' fluctuations in reading motivation reflect the way in which students interact socially, personally, and materially with reading activities irrespective of the content area of classrooms. These findings provide guidance on how an interactive approach to reading motivation can be integrated into the motivation literature for improving instruction for adolescent readers and extends pedagogical and empirical research on supporting environmentally triggered reading motivations.