This study investigated the effects of beta2-adrenergic agonist therapy on heart rate variability (HRV) in adult asthmatic patients by using frequency domain measures of HRV. A randomized crossover design was used. Twenty adult patients with asthma were studied. All patients showed a mild-to-moderate decrease in baseline forced expiratory volume in one second. Any diseases that might have influenced the autonomic function were excluded. All patients had a complete physical examination and medical history that revealed no cardiovascular disease or medication. The study used 200 microg inhaled salbutamol and 500 microg inhaled terbutaline. HRV analysis was performed for each 5-min segment, 5 min before inhalation of the study drug and 5, 10, 15, 20, 25 and 30 min after inhalation. Total power (TP: <0.40 Hz), high-frequency power (HF: 0.15-0.40 Hz), low-frequency power (LF: 0.04-0.15 Hz) and LF/HF ratio were calculated. The LF and LF/HF ratio increased and TP decreased at 5, 10, 15 and 20 min after the salbutamol and the terbutaline inhalation, HF did not change significantly after the salbutamol and terbutaline inhalation. Acute salbutamol and terbutaline inhalation produce similar effects on heart rate variability and increase sympathetic modulation in the cardiac autonomic activity.