1. Asthmatic patients bronchodilate in response to infused atrial natriuretic factor. We wished to determine whether the airways of normal subjects responded in a similar way.

2. Changes in airway resistance, as determined by specific airway conductance, were measured in eight normal subjects in response to intravenous infusion of atrial natriuretic factor at doses of 0.5, 2 and 10 pmol rnin−1 kg−1.

3. No significant effect was observed on specific airway conductance at any rate of infusion despite maximum mean (sem) plasma levels of 597 (62) pg of atrial natriuretic factor/ml in peripheral venous blood.

4. A second study was performed using six of the eight original subjects and employing a pharmacological dose of 50 pmol of atrial natriuretic factor min−1 kg−1. This produced mean plasma levels of 2056 pg/ml and a mean increase of 31% in specific airway conductance.

5. It is concluded that pharmacological, but not pathophysiological, elevations of plasma atrial natriuretic factor may significantly alter bronchomotor tone in normal subjects.

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