This study investigates consistency of 11th grade students' ideas about the concept of rate across different contexts namely velocity of an object in physics and rate of reaction in chemistry. The subjects of the study were 200 11th grade high school students in three schools. Data were collected after formal teaching on reaction rates and kinematics. Data sources included students' responses to six diagnostic questions and interviews. The results suggest that consistency of students' ideas about rate concept across different contexts and domains are limited. The results revealed that students' ideas about the concept of rate depend to some extent on the format and contextual features of the questions presented. It is observed that students are more likely to give a correct answer when data are presented on a table rather than on a graph. There is very little evidence showing that students coherently apply their ideas about rate concept across a wide range of contexts. Implications for designing instruction that would provide students with opportunities to develop metacognitive skills are suggested. In this respect, teachers from different disciplines can cooperate with each other when they teach closely related concepts in biology, chemistry, physics or mathematics.