1. Stroke volume and cardiac output were measured using the Doppler ultrasound technique in 16 normal subjects immersed to the neck in water at 33 degrees C, 35 degrees C, 37 degrees C and 39 degrees C. A standard aortic diameter was assumed and results were expressed as percentage changes from pre-immersion resting values.

2. Cardiac output rose progressively at higher temperatures, increasing by 30% at 33 degrees C and by 121% at 39 degrees C. At thermoneutral temperatures (33 degrees C and 35 degrees C) this was achieved by an increase in stroke volume of 50% despite a significant decrease in heart rate. There was a further rise in stroke volume and pulse rate at higher temperatures and a mean tachycardia of 109 +/- 4 beats/min was noted at 39 degrees C. Calculated peripheral resistance reduced progressively with increasing temperature of immersion.

3. This non-invasive and simple technique may provide a non-exercise-related cardiovascular stress test to study cardiovascular responses in a variety of pathophysiological states.