We present time series of tritium (3H) concentrations in seawater from stations in the eastern subpolar (50¿N, 145¿W) and subtropical (28¿N, 122¿W) North Pacific. In the eastern subpolar North Pacific the tritium gradient between surface water and North Pacific Intermediate Water is smallest during years that coincide with El Ni¿o events. In the eastern subtropical North Pacific between 200 and 400 m the tritium signature suggests that the water is of subpolar origin during non-El Ni¿o years, the water at this location is devoid of tritium. We hypothesize that the El Ni¿o phenomenon alters both the ventilation of thermocline and intermediate waters in the eastern subpolar North Pacific, as well as the transport of this water to the eastern subtropical gyre. Combined with satellite altimeter data, these results offer both a mechanism and a time frame by which subpolar water ventilates the vast reservoir of the subtropical North Pacific. ¿ American Geophysical Union 1993