'The secular variation in the tritium content of precipitation, particularly in the polar regions, is investigated by mathematical treatment of a compartment model of exchangeable atmospheric reservoirs. Exchange coefficients are calculated to be 0.70 per year and 0 35 per year for interstratospheric exchange and exchange between stratosphere and troposhpere, respectively. The stratospheric half-live of bomb-produced tritium amounts to 20 months. A special Antarctic continental effect is discussed.