In this study, both acute electrophysiologic actions of intravenously administered theophylline and clinical effects of chronic oral theophylline therapy were assessed in 10 young patients (aged 9 to 41 years) without clinically significant cardiac disease, in whom recurrent symptoms of syncope and dizziness were attributed to transient bradyarrhythmias (sinus pauses, marked sinus bradycardia or paroxysmal atrioventricular [AV] block). Intravenous theophylline infusion (serum concentration range 9.5 to 12.0 mg/liter) shortened means sinus cycle length (control 973 ± 285 ms versus theophylline 880 ± 226 ms, p <0.005) and decreased both the estimated sinoatrial conduction time (control 169 ± 56.0 ms versus theophylline 143 ± 55.3 ms, p <0.05) and the maximum corrected sinus node recovery time (control 442 ±251.0 ms versus theophylline 255 ± 146.2 ms, p <0.05). In addition, theophylline infusion shortened the minimum atrial paced cycle length with sustained 1:1 AV conduction (control 414 ± 86 ms versus theophylline 379 ± 97 ms, p <0.05) and consistently reduced AV node functional refractory periods. Subsequent chronic oral theophylline therapy (serum levels 9 to 12 mg/liter) was tolerated in 8 patients (80%). During a follow-up of 5 to 24 months, suppression of symptoms was achieved in 6 of the 8 patients. Thus, theophylline exhibits positive chronotropic and dromotropic effects in man at serum concentrations in the usual therapeutic range (10 to 15 mg/liter). Furthermore, suppression of symptoms during follow-up suggests that theophylline treatment may be a useful therapeutic consideration in some patients with recurrent symptomatic bradyarrhythmias.