This study was undertaken to investigate whether or not substrate selection during exercise is altered with participation in Ramadan, and whether or not this alteration is influenced by exercise intensity. Eight men (21-41 years) exercised on an electronically braked cycle ergometer at three 10-min workloads (45, 60 and 75% VO(2peak)): (1) the week before Ramadan; (2) the end of the first week of Ramadan and (3) the final week of Ramadan. Four subjects were unable to complete the final 10-min (75% VO(2peak)) workload during Ramadan. During the two lower workloads, exercise RER significantly decreased during Ramadan (p<0.01) and there was a significant effect of Ramadan on the increase in RER with increased workload (p=0.041). Post hoc contrasts revealed only that RER during exercise at the end of the first week was significantly lower (p<0.01) than pre-Ramadan. Rate of lipid oxidation increased from 0.18gmin(-1)+/-0.22 to 0.31gmin(-1)+/-0.28 by the first week of Ramadan (p<0.01), the effect reversed by the final week (0.23gmin(-1)+/-0.22) (p=0.02). Although body weight declined during Ramadan (from 71.1kg+/-6.9 to 69.8kg+/-7.3, p=0.02), percentage body fat, measured via underwater weighing, did not change. In conclusion, daily fasting during Ramadan induces changes in substrate selection during submaximal exercise within 1 week, but these changes are moderated with continued daily fasting. However, changes in anthropometric variables reflect a net energy deficit during Ramadan which may have mediated the observed increased lipid oxidation during submaximal exercise.