During the 1975 and 1976 International Weddel Sea Oceanographic Expeditions, samples were collected for analysis of the Mount Soledad Tritium Laboratory. Most samples were of near-bottom water in the western and nothern Weddell Sea. Some samples were also collected of Warm Deep Water, Winter Water, and surface water. Highest tritium concentrations in bottom water were found in the northern Weddell Sea and along the western continental shelf. This indicates that the most recently formed bottom water is in these areas. In the central Weddell Sea, concentrations were lower, indicating slower renewal of bottom water. An upper limit of 3.5¿106m3/s is obtained for the formation rate of Weddell Sea Bottom Water. This yields a production rate of about 7¿106m3/s for classical Antarctic Bottom Water. Surface water and Winter Water also showed a strong gradient in tritium concentrations between the central Weddell Sea and the continental shelf.