Continuous portable 24 hour tape recorded monitoring of the electrocardiogram in 400 randomly selected adults was used to provide data about arrhythmia during normal daily activity. The sample contained 203 females and 197 males of which 100 were Maori and 300 European. The ages ranged from 20 to 70 years and the mean was 45.1. The only exclusion was inability to wear a Holter monitor. Arrhythmia, mainly asymptomatic, occurred in 318 (80 percent). There was equal prevalence in Maori and European. In 43 (13.5 percent) it was ventricular, in 122 (38.4 percent) it was supraventricular, and in a further 153 (48.1 percent) it was combined. In 45 (14.1 percent) ventricular ectopy was considered potentially serious. Physical abnormality of some description, excluding arrhythmia, was present in 90 percent of Maoris and 62 percent of Europeans. Arrhythmia was slightly less prevalent in totally normal subjects than in those considered to have some abnormality (p = less than 0.05).