Panting is the major avenue of evaporative cooling in dogs exposed to heat and/or exercise. We found modulation of evaporation was achieved by varying the paths of airflow during inhalation and exhalation. The direction of airflow through the nose and mouth was determined by measuring pressure changes and temperature at the openings of one nostril and the mouth in three dogs (av. weight 22 kg). Rates of oxygen consumption and respiratory evaporation were measured simultaneously. Three patterns of panting were observed as the demand for respiratory evaporation increased: (I), inhalation and exhalation through nose; (II), inhalation through nose, exhalation through nose and mouth; and (III), inhalation through nose and mouth, exhalation through nose and mouth. Pattern I was observed in resting dogs when ambient temperature was below 26 degrees C and when animals ran at slow speeds in the cold (e.g. 0.8 m . s(-1) at 10 degrees C). Patterns II and III were observed when dogs rested quietly at ambient temperatures above 30 degrees C and during exercise except when dogs ran slowly at very low temperatures. Patterns II and III rarely occurred independently for long periods of time. Instead, there was normally a continual oscillation between the two. The proportion of the time that the dog used Pattern III instead of Pattern II increased as temperature and/or speed were increased; but the correlation between rate of respiratory evaporation and the percentage of time Pattern III was utilized was weak (r2=0.63).