The quality of classroom interactions has typically been studied using aggregates of ratings over time. However, within-day ratings may contain important variability. This study investigated within-day variability using the NICHD Study of Early Childcare and Youth Development's observational data during grades 3 and 5. The first question examined the stability of classroom interaction quality within a day, and the second question examined the extent to which time of day, activity setting, numbers of adults and children, and instructional grouping predicted the quality of classroom interactions within a day. Results indicated that classroom interactions were most stable within emotional support dimensions and least stable within instructional support dimensions. Results also indicated that the start of the day and transitions were associated with lower-quality classroom interactions. Academic activities, more teachers, fewer students, and group instruction were associated with higher quality. Implications are discussed in terms of understanding students' experiences and conducting classroom observations. (Contains 1 figure, 5 tables, and 1 note.)