Passive elevation of the body core temperature (Tc) induces rapid, shallow breathing in resting man. We wondered if exercise-induced Tc elevation would also lead to decreased tidal volume (VT) and increased breathing frequency (f) during exercise. To investigate this question, 10 subjects each performed 47 min of cycle ergometer exercise at 50--60% of the maximal aerobic capacity, with the work rate adjusted to maintain ventilation (VE) constant. This long ride raised mean Tc (rectal) 0.8 degrees C. Before and immediately after the long ride, ranges of VE and VT were obtained from short 6-min rides that progressed from unloaded pedaling to the anaerobic threshold. At the constant VE of the long ride, f rose and VT fell as Tc rose (P less than 0.05). The fall in VT was associated with a fall in inspiratory time (TI); drive (VT/TI) and timing (TI/Ttot)components of VE were unchanged. These effects were consistent over the entire range of VE obtained from the short 6-min rides. Passive heating in warm water to produce equal Tc elevation in the same subjects yielded similar exercise breathing-pattern changes. These findings suggest that increased Tc mediates the VT fall during prolonged exercise, possibly through stimulation of the central respiratory pacemaker.