Recharge to the Quaternary aquifers beneath the Northern Adelaide Plains was estimated to be 3.9 x 106m3/year using the environmental tritium concentration of water samples taken from suitably sited boreholes. Recharge to the Tertiary aquifers was estimated to be 0.4 x 106m3/year, by the same isotopic techniques. These results may be compared with a total recharge by leakage through the beds of streams which cross the plain, estimated by the South Australian Department of Mines to be 6.4 x 106m3/year.