The 3H-3He age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere. Between 1992 and 1995 a detailed study of 3H-3He ages was conducted in Lake Baikal, the deepest and largest lake by volume on Earth, to investigate deep water renewal in its three major basins. Maximum 3H-3He ages are 14--17 years in the southern basin, 16--18 years in the central basin, and 10--11 years in the northern basin. Rates of renewal of deep water with surface water, deduced from volume-weighted mean 3H-3He ages below 250 m depth, are about 10% yr-1 in the southern and central basins and 15% yr-1 in the northern basin. In the southern basin the mean 3H-3He age below 250 m depth increased steadily from 9.6 years in 1992 to 11 years in 1995, indicating a slight diminution in deep water renewal during this time. Bottom water renewal by large-scale advection was estimated from the mass balance of 3He in the 200 m thick bottom layer of each basin. Bottom water renewal rates in the northern basin were found to be between 80 and 150 km3 yr-1 and in the central basin between 10 and 20 km3 yr-1, whereas in the southern basin they were practically zero. Correlating oxygen and dissolved helium-4 concentrations with 3H-3He age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column (4.5 μmol L-1 yr-1), as well as mean helium fluxes from the lake bottom (2.8¿1011 atoms m-2 s-1 in the northern basin, and 1.3¿1011 atoms m-2 s-1 in the central and southern basins). The helium isotope ratio of the terrigenic helium component injected from the lake bottom, determined from measurements of water from hydrothermal springs in the vicinity of the lake, was found to be ~2.2¿10-7. ¿ 1998 American Geophysical Union