Routine electrocardiographic tracings (ECGs) were obtained from 491 male and 236 female Norwegians. The atrioventricular conduction time (P-R interval) was read independently on coded ECGs by two physicians. The frequency distributions are presented: mean +/- 2 SD was 0.16 +/- 0.04 s and 0.15 +/- 0.04 s for males and females, respectively. Intra-individual repeatability over 1 year was high, and the measurement error was small. Within each sex, body size, age, heart rate and non-cardiac disease manifestations had little influence on the P-R interval. The P-R interval may be reliably determined in routine ECG tracings by trained observers.