To investigate the effect on the pattern of breathing of cooling receptors in the nose, eight normal male volunteers underwent a steady-state CO2 stimulation by nasal inhalation. The inhaled gas temperature was randomly switched between "warm" (32 degrees C) and "cold" (2 degrees C) at each of three levels of inspired CO2 fraction (FICO2). Breathing cold air through the nose reduced the mean slope of the ventilatory response to CO2 by 27% (P less than 0.05) and the mean intercept at PCO2 of 45 Torr by 6.6 l/min (P less than 0.01). This was due mainly to a reduction in tidal volume (VT). Analysis of the breathing pattern recorded at a high level of minute ventilation (VE) (end-tidal partial pressure of CO2 approximately 52 Torr) showed a reduction of VE that was due almost entirely to a reduction in VT (P less than 0.05) associated with a reduction in inspiratory time (TI) as a fraction of total respiratory cycle time (P less than 0.05) but little change in VT/TI. In a separate experiment conducted with five subjects, there was no significant difference in inspired nasal resistance between warm and cold runs during CO2-stimulated breathing. The results confirm the previous observation that cold air breathed through the nose inhibits ventilation in normal subjects and show that this is not related to an increase in flow resistance. The reduction in ventilation is due to reduction in VT associated with shortening of the duty cycle.