The bronchodilating effects of atrial natriuretic factor (ANF) were compared to effects of salbutamol in a double-blind, placebo-controlled, crossover study in eight subjects with asthma and with airflow limitation. ANF was infused intravenously (0.1 microgram/kg/min during 30 minutes) and caused a short-lasting significant (p less than 0.012) bronchodilation in all patients similar in intensity to intravenous salbutamol (0.13 microgram/kg/min). The duration of the effect of the beta 2-agonist, however, was longer. No severe side effects were noted with ANF infusion. ANF may play a role in the modulation of the bronchial tone in humans, but its usefulness in the treatment of asthma remains to be determined.