Two cases of atrial parasystole showing the various manifestations of the arrhythmia are presented. Analysis of the underlying mechanisms shows that atrial parasystolic bigeminy with `reversed' coupling is a form of escape-capture begeminy, sinus escapes being followed by an ectopic capture of the atria. Reasons are given for the rarity of atrial fusion beats. The similarities and differences between atrial and ventricular parasystole are explored. It is suggested that an atrial parasystolic pacemaker may lie within a major atrial preferential conducting pathway, and may consist of a congenitally ectopic fragment of sinus nodal tissue. The clinical significance of the arrhythmia is discussed; the associated diseases apparently represent a cross-section of medical ward experience.