Nitric oxide (NO) is an important biological mediator with a variety of cellular and tissue functions. Fractional exhaled (FE(NO)) and nasal NO (FN(NO)) are known to be altered in several respiratory tract disease and disorders. However, there are rather few studies into the reproducibility of FE(NO) or FN(NO) measurements in healthy subjects. The aim of this study was to examine both short- and long-term variations of FE(NO) and FN(NO). In addition to intraday, day-to-day and week-to-week variations, the seasonal variation was assessed. The data revealed that FE(NO) and FN(NO) levels were lower in the mornings compared to the afternoon values. There were clinically significant correlations in day-to-day (morning values 121.4+/-37.5 vs. 119.8+/-29.6, r=0.868, P=0.000), week-to-week (morning values 121.4+/-37.5 vs. 128.0+/-32.1, r=0.637, P=0.000) and seasonal (winter 133.5+/-29.7, summer 138.1+/-47.6, autumn 121.4+/-37.5, r=0.624, P=0.000) values of FN(NO). Day-to-day (morning values 14.6+/-6.7 vs. 14.2+/-5.1, r=0.784, P=0.000) week-to-week (morning values 14.6+/-6.7 vs. 15.4+/-7.3, r=0.738, P=0.000) and seasonal (autumn 14.6+/-7.2, winter 16.6+/-6.4, summer 17.4+/-8.0, r=0.709, P=0.000) levels of FE(NO) were also highly reproducible. Serial FE(NO) and FN(NO) measurements can be used in the monitoring of respiratory tract inflammation. Due to the diurnal variation of FE(NO) and FN(NO,) in long-term follow-up the measurements should be performed at the same time of day.