The distribution of tritium in the North Atlantic in 1972 is compared with the distribution of salinity and the first-order potential vorticity as mapped on six constant potential density surfaces in the North Atlantic. The picture presented suggests an advective transport regime which is consistent with currently developing notions of the large-scale gyre structure. Lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre, and vertical (cross-isopycnal) mixing needs to be invoked only in near surface layers.