The purpose of this study was to evaluate metabolic and circulatory responses to interval training of legs or arms during steady-state, submaximal cycling. 15 college males cycled on a bicycle ergometer twice with arms (63 and 83 W) and twice with legs (100 and 125 W) before and following 5 weeks of daily interval training. Seven subjects trained with arm cycling and eight with leg cycling. Significant post-training decreases in submaximal oxygen consumption (VO2), heart rate (HR), and venous blood lactate (LA(V)) were noted when cycling with trained and untrained muscles with the magnitude of change significantly greater with trained muscles. These results indicate metabolic and circulatory adaptations to interval training that are mediated centrally and peripherally. With respect to HR, but not VO2, training a larger muscle mass (legs) produced a greater central but lesser peripheral effect whereas the opposite was true for the smaller arm muscles. The data also suggested that the peripheral adaptation involves a common mechanism controlling both HR and LA(V)) changes with a separate mechanism controlling VO2.