Water mass evolution and ventilation in warm-core rings of the Kuroshio is discussed using hydrographic (T-S) data, remote sensing, and tritium measurements. The Kuroshio warm-core ring A3 detached from the Kuroshio in 1986, migrated northwest along the Japan and Kuril trenches, and reached the southern Kuril Islands in 1989. This ring is characterized by weak stratification to a depth of about 700 M. In its upper layer the ring displayed a pronounced thermostat and a high tritium concentration; in its lower portion, however, temperature and salinity were density compensated. On the basis of tritium data analyses, the thermostat water was shown to originate in intermediate layers of the Kuroshio south of the subarctic front. This implies tht the warm-core rings are a major factor influencing the tritium transport in this region and that they are an important link for water exchange between the subtropical and subpolar gyres in the North Pacific. The tritium data confirm the idea about the origin of the thermostad water of anticyclonic ring from intermediate layers of the Kuroshio. Therefore a mechanism must exist in the Kuroshio frontal zone, which selectively draws water from the Kuroshio to the rings and acts to ventilate the subtropical gyre. It is also noted that during the ring cooling period a surface layer of low tritium content was formed above the ring. ¿1991 American Geophysical Union