111 healthy subjects, 57 males and 54 females, 20-79 years of age, randomly selected among the participants of the Copenhagen City Heart Study were tested for ventricular ectopic activity using ambulatory 24-h ECG recording. One or more premature ventricular beats (PVBs) appeared in 68 subjects (61%), 35 males and 33 females (61% of each group). The prevalence of PVBs increased significantly with age: 31, 68 and 84% in the respective age groups: 20-39, 40-59 and 60-79 (P less than 0.05). The number of PVBs per 24 h was less than 24 in 88 (79%), less than 300 in 105 (95%), and less than 500 in 107 (96%). More than 1000 PVBs per 24 h appeared in 3 (2.7%), one of whom had 11 669. The number of PVBs per 24 h increased with age, but the increase was not statistically significant. The number of PVBs per 100 000 recorded QRS-complexes did not differ significantly from the number of PVBs per 24 h. The number of PVBs h-1 was highest in daytime, and apparently higher in males than in females. However, the difference between the two sexes was not statistically significant. Multiform PVBs appeared in 4 subjects, interpolated PVBs in 2, a run of 5 PVBs in 1, dropped beats in 3, two pauses of 1900 ms duration in 1, and several post-acceleration pauses of 1300 ms duration in 1.