During the period 1959--1973 the La Jolla Tritium Laboratory monitored bomb-produced tritium in surface seawater from 15 stations in the Pacific Ocean. Some 500 tritium determinations were carried out during this time. Furthermore, between 1965 and 1972 a series of subsurface samples from 49 locations in the Pacific Ocean was measured, and tritium concentrations as a function of depth were determined. The determinations had a standard error of ¿4% and an uncertainty in the zero value of about 0.17 tritium unit. The most significant oceanographic conclusions that must be drawn from these data can be summarized in the following way: (1) Lateral flow along contours of constant density is the major factor controlling the tritium distribution in the surface and subsurface waters of the Pacific Ocean. Input of tritium into the water below the mixed layer occurs between 45¿ and 30¿N and south of 60¿S. The sinking of water in the mid-Pacific at 30¿N and its flow toward the equator was previously recognized from the presence of a salinity maximum below the thermocline. Our tritium measurements allow us now to estimate the southward component of the rate of this flow to be 1.3¿0.5 cm/s. In many other areas the tritium content of water under the mixed layer is undoubtedly also due to local sinking of surface water and subsequent lateral subsurface flow. (2) An upper limit for the coefficient of vertical eddy diffusion can be calculated from our tritium data in the eastern North Pacific gyre. This upper limit of 0.3 cm2/s is well below estimates derived by most other methods (Veronis, 1969), but it agrees well with the upper limit of 0.2 cm2/s derived by Rooth and ¿stlund (1972) from tritium measurements in the Sargasso Sea. Thus it appears that transport across lines of constant density is much slower than previously assumed. (3) The mean lifetime of water in the mixed layer of the North Pacific Ocean, i.e., the residence time, varies greatly with geographical location. North of 10¿N the average residence time is between 9 and 15 years, depending on the assumed rate of the molecular exchange of water between atmosphere and ocean.