Inhaled beta-stimulants, which have arrythmogenic effects, are widely used as a premedication when asthmatic children take swim exercise. Recently, some authors have reported that cardiac arrhythmias have been observed in greater frequency during swimming and diving. To elucidate the possibility that inhaled beta-stimulants induce more frequent arrhythmias during swimming or diving, electrocardiograms were recorded during swimming and apneic diving, with and without an inhaled beta stimulant (isoproterenol). During swimming, electrocardiograms were recorded from thirteen asthmatic children of mean age (±SD) 8.8±6.3 years, with and without isoproterenol inhalation. With isoproterenol inhalation, a higher heart rate was observed just before, during and one minute after swimming. During apneic diving, electrocardiograms were recorded from ten asthmatic children of mean age 10.9±3.5 years, with and without isoproterenol inhalation. Without isoproterenol inhalation, four arrhythmias were observed, while three arrythmias from one patient were observed with isoproterenol inhalation. With isoproterenol inhalation, a smaller decrease in heart rate was observed during diving. From these observations, it is concluded that isoprotereol inhalation as a premedication is not likely to increase the frequency of arrhythmias during swimming or diving. Moreover, arrhythmias associated with increased vagal tone may be suppressed by isoproterenol inhalation.