A huge amount of radioactive 129I has been released into the environment from the nuclear energy industry, atomic weapon tests, and nuclear accidents. In this study, we present weekly and seasonal data on 129I measured in precipitation and runoff of northern Europe during 1998 and 1999. The 129I concentration is at 10(8)-10(9) atoms/L in precipitation and (2-5) x 10(8) atoms/L in runoff water, and it is 3-4 orders of magnitude higher than in the prenuclear era. Snow shows lower 129I concentration than rain, and there is apparently a positive correlation between surface air temperature and 129I. Precipitation chemistry, expressed as the content of Cl, SO4, and NO3 and atmospheric ozone, exhibits weak negative correlation with 129I values. Our 129I data on precipitation suggest significant influence of the northern European atmosphere by the discharges from the nuclear reprocessing facilities at Sellafield and La Hague.