The aim of the study was to investigate the relationships between situational and contextual self-determined motivation and physical activity intensity as measured by heart rates during a ninth-grade students' physical education (PE) class. The participants of the study were 139 Finnish ninth-grade students (15-year-olds). The data were collected during one 60-minute PE class held by the class PE teacher. Results revealed that more self-determined situational motivation negatively predicted the lowest heart rate category (less than 120 beats/min.) as well as the next lowest heart rate category (120-40 beats/min.). Neither variation in type of situational or contextual motivations explained heart rates in the next highest category (140-60 beats/min.), whereas the highest heart rate category (greater than 160 beats/min.) was positively predicted by situational motivation. Lastly, highly self-determined situational motivation accounted for a significant amount of variance in mean heart rate value. Taken together, results revealed that situational motivation associated more strongly with physical activity intensity as measured by heart rate than did contextual motivation. (Contains 4 tables.)