To determine the relationship between the training-induced increases in plasma volume (PV) and alterations in cardiac performance during prolonged submaximal cycle exercise, seven male subjects were studied prior to and following a short-term (3 d) training period (2 h.d-1 at 65% VO2max). Mean (range) VO2max was 3.42 l.min-1 (2.96-3.87). Training resulted in a 20% increase (P less than 0.05) in plasma volume (PV) and a 12% increase (P less than 0.05) in total blood volume (TBV). In contrast, training had no effect (P greater than 0.05) in altering exercise VO2, VCO2, VE BTPS, or RER. Cardiac output (Q) was higher (P less than 0.05) posttraining at all exercise sampling times (30, 60, 90, and 120 min). The elevations in Q were accompanied by an average decrease (P less than 0.05) in stroke volume (SV) of 22 ml. Arteriovenous O2 (a-v O2) difference was depressed (P less than 0.05) during exercise following the training. Although elevations (P less than 0.05) in core temperature (degrees C) occurred during the exercise, the training-induced PV increases did not affect thermoregulatory behavior. These results indicate that an early adaptive response to exercise training is an elevation in Q, an increase in SV, and a reduction in HR. These effects persist during prolonged exercise in spite of the progressive increase in body heat content. It is proposed that the increase in Q serves primarily to increase muscle blood flow and maintain arterial O2 delivery, while the altered cardiodynamic behavior serves to increase cardiac reserve, providing a greater tolerance to prolonged heavy exercise.