The influences of vagal and sympathetic efferent activity on sinus arrhythmia in man have been studied in healthy subjects by administration of hyoscine butylbromide and atenolol alone and combined using a microcomputer-linked electrocardiogram (e.c.g.) system. Sinus arrhythmia was quantitated as the S.D. of the R-R interval. Sinus arrhythmia was reduced by hyoscine butylbromide, in some subjects to near abolition, but this end-point was unchanged by pre-treatment with atenolol. Atenolol alone prolonged the mean R-R interval and increased sinus arrhythmia. It is suggested that sinus arrhythmia in man is mediated through vagal efferents alone but that atenolol increases the arrhythmia through a central vagotonic effect.