From the tritium to salinity relationship on stations occupied in early 1974 on both sides of the Strait of Gibraltar, a mean tritium concentration of 1.6¿0.2 TU (1 sigma) is derived for the original Mediterranean water that feeds the outlow. The same tritium concentration is observed in the Mediterranean near to the Strait (Alboran Sea) in about 700 m depth. The layers shallower than about 450 m have so much higher tritium concentration that it can be concluded that they contribute only a small fraction of the outflow.