This study compares the effects of the Ramadan fast (RF) on body and plasma compositions, hematology, and responses to steady state submaximal exercise in six physically active (A, 35.5+/-1.6 y) and seven sedentary (S, 37.6+/-2.3 y) Kuwaiti men. Subjects were evaluated: 1) 1 wk before RF (pre-RF); 2) 2 wk after the start of RF (mid-RF); 3) at the 4th wk of RF (end-RF). Total body weight and body fat decreased insignificantly (<1.5%, P>0.3) in both groups. At the end of Ramadan, significant increases in osmolarity (P<0.05), Na (P<0.05) and bicarbonate (P<0.05), and a decrease in serum iron were noted in sedentary but not in active subjects. Platelet count was lower in both groups by the end of RF (P<0.05). Body weight and percent fat changed little (<1.5%) in either group. During RF, submaximal exercise heart rate declined insignificantly in group S but significantly (P<0.05) in group A. The respiratory exchange ratio R during steady state submaximal exercise decreased markedly (P<0.001) by the end of RF in both groups. It is concluded that energy balance is well maintained during RF both in S and in A subjects. Metabolic adaptations during RF result in lower exercise R due to increased lipid usage. Deficits or redistribution of specific micronutrients (iron, vitamins) may account for reductions in serum iron and platelet counts, particularly in sedentary subjects that need to limit intake to maintain body weight. The decrease in submaximal exercise heart rate indicates that during RF, cardiovascular adaptation to conditioning is adequate in the more physically active group. Body fluid balance was better maintained in active than in sedentary subjects.