The physical activity levels of 47, 5- to 7-year-old children were assessed before and after a school playground was painted with fluorescent markings. Children‘s physical activity was measured using heart rate telemetry during three playtimes before and after the markings were laid down. Children in the experimental and control groups spent 27 and 29 min, respectively, in moderate to vigorous physical activity (MVPA) before the intervention, increasing to 45 and 36 min, respectively, during the intervention period. MVPA, vigorous physical activity (VPA) and mean heart rate remained relatively stable in the control group compared to respective increases of 10 and 5% of playtime and 6 beats min ^{- 1} in the experimental group during the intervention period. The ANCOVA analysis revealed significant interactions and main effects for the intervention for MVPA, VPA and mean heart rate. Conversely there were no main effect differences between groups. These results suggest that while playground markings had a significant and positive influence on children's physical activity, factors other than playground markings may also influence children's physically active play.