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1. Continuous recordings of the arterial blood pressure and intrathoracic pressure of conscious dogs have been taken, in each animal, under two different conditions: (a) during mild exercise on a treadmill in a thermoneutral environment (19-22° C T_db, 13·5-17·5° C T_wb); (b) at rest in conditions of mild heat stress (35-40° C T_db, 20-26° C T_wb), and of severe heat stress (35-40° C T_db, 30-34° C T_wb).

2. Under all conditions, panting occurred, but there were differences in its pattern. During exercise, panting usually occurred in short regular bursts of about 10 sec duration, whereas during both mild and severe heat stress it occurred in bursts of irregular but usually longer duration. At comparable rectal temperatures, the intensity of panting (as measured by the amplitude of intrathoracic pressure variations) was markedly greater during exercise than during heat stress; the frequency, however, was lower.

3. During exercise, the bursts of panting were associated with striking oscillations in mean blood pressure, whereas during mild and severe heat stress they were associated with much smaller changes in mean blood pressure. In addition, cross-correlation analysis revealed that blood pressure changes were far more strongly correlated with bursts of panting during exercise than during mild or severe heat stress.

4. In view of the present and previous results, it is suggested that both the bursts of panting and the associated cardiovascular changes during exercise are stimulated primarily by metabolic rather than thermoregulatory factors. During heat stress, bursts of panting are solely thermally induced, and are therefore not associated with similar cardiovascular alterations.