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Theophylline increases sinus rate, but as yet its use has not been investigated in patients with chronic atrioventricular conduction disturbances. Resting electrocardiogram, 24-h Holter recording and treadmill test were performed in 17 patients with chronic atrial fibrillation and a slow ventricular response not related to drugs (age: 75 +/- 8 years). Then slow-release theophylline was administered (700 mg daily) and after 5 days these investigations were repeated with the same methods. Theophylline increased mean resting heart rate (51 +/- 6 versus 67 +/- 13 beats.min-1, P less than 0.01), mean 24-h heart rate (51 +/- 6 versus 68 +/- 14 beats.min-1, P less than 0.01) and minimal 24-h heart rate (32 +/- 6 versus 42 +/- 11 beats.min-1, P less than 0.01). Cardiac pauses greater than 2.5 s were present in 13 patients during control recording; after theophylline they disappeared in 11 and markedly decreased in the remaining two. The longest R-R interval decreased in all patients (3218 +/- 943 versus 2121 +/- 518 ms, P less than 0.01). The daily number of wide QRS complexes increased in 16 out of 17 patients (428 +/- 752 versus 1146 +/- 1464 ms, P less than 0.01). Exercise heart rate, evaluated at the end of first and second stage, was higher after theophylline than during control test (P less than 0.01). These data suggest that oral theophylline can represent a valid therapy in most patients with atrial fibrillation and a slow ventricular response.