The dynamic QT relationship between the QT and RR intervals in normal individuals, including sex differences, has not been well examined. The aim of this Holter monitor-based study was to assess circadian and sex-related variations in QT dynamics in healthy subjects. The study population consisted of 50 healthy volunteers (mean age = 32 +/- 6 years, 25 men), in whom 24-hour digital Holter monitoring and QT interactive, beat-by-beat analyses were performed. The mean lengths of QT and RR intervals were measured from the 24-hour recordings. In order to assess QT dynamics, QT/RR linear regression was performed, and the slope was calculated over 24 hour and for day and night periods, and both genders separately. In the whole population, the mean QT interval was 356.5 +/- 19.2 ms and RR interval was 785.9 +/- 80.7 ms. The mean value of the slope over 24 hour was 0.17 +/- 0.03, though significantly steeper during the day (0.13 +/- 0.03) than at night (0.09 +/- 0.03, P < 0.001). The analysis of QT/RR dynamics over 24 hour revealed a significantly steeper slope in women (0.18 +/- 0.03) than in men (0.16 +/- 0.03, P = 0.006), as well as during daytime (0.14 +/- 0.03 vs 0.12 +/- 0.03, P = 0.04). Circadian variations and sex differences were observed in QT dynamics. The latter may explain the greater susceptibility of women to torsades de pointes during treatment with drugs that prolong repolarization.