In an upper-division, college course with a lecture component and two laboratory sections, we experimentally evaluated a treatment package that included this contingency: "only if students attended lecture and submitted notes for each day's reading assignment could they use their notes during a later test," and instructions about the contingency. We examined whether the instructed contingency enhanced: (a) students completing notes on reading assignments before lecture and (b) their attending lecture. Although the instructed contingency improved these behaviors, improvement depended on the semester we conducted our experiment and the students' laboratory section. The instructed contingency was, however, most helpful where most needed: for the laboratory whose students had the lowest attendance rates at lecture. For these students the instructed contingency--a non-punitive, inexpensive intervention--enhanced preparation for and attendance at lecture across two experiments and appeared to support such behavior in subsequent offerings of the course. (Contains 2 figures.)