The previous studies relating to the effect of the Valsalva maneuver on P wave do not provide detailed information. In those studies, the parameters related to P wave duration during the release phase of the Valsalva maneuver were evaluated. The authors evaluated P wave amplitude as well as P wave duration during the strain phase of the Valsalva maneuver. Thirty-seven normal subjects and 36 patients with paroxysmal atrial fibrillation (PAF) were included in the study. Twelve-lead surface electrocardiography (ECG) was obtained from all the patients before and during the strain phase of the Valsalva maneuver. The authors evaluated the parameters related to both P wave duration and P wave amplitude. The highest P wave voltage was expressed as the P wave amplitude maximum (P amp max), the lowest P wave voltage as the P wave amplitude minimum (P amp min), and the P wave amplitude dispersion (P amp dispersion) as "the P amp max - the P amp min." Also, the maximum P wave duration was expressed as the P maximum (P max), the minimum P wave duration as the P minimum (P min), and the P wave dispersion as "the maximum P wave duration (P max) - the minimum P wave duration (P min)." All these parameters were measured before and during the strain phase of the Valsalva maneuver. P max, P dispersion, P amp max, and P amp dispersion were higher in patients with PAF compared to those of normal subjects before the Valsalva maneuver. In normal subjects, P max, P wave dispersion, P amp max, and P amp dispersion values were significantly higher in the strain phase of the Valsalva maneuver than before (p<0.01, p<0.01, p<0.05, and p<0.05, respectively). In the patients with PAF, P max, P wave dispersion, P amp max, and P amp dispersion values were significantly lower during the strain phase of the Valsalva maneuver than before. In conclusion, in the patients with PAF, P wave parameters were decreased during the strain phase of the Valsalva maneuver, approaching to the normal levels.