Compositions of basement fluids are presented for four sites along a 3.5-m.y.-old, partly buried basement ridge on the eastern flank of the Juan de Fuca Ridge. This ridge is roughly parallel to the active ridge axis of the Endeavor Segment ~100 km to the west. From south to north these sites are Baby Bare Outcrop, Ocean Drilling Program (ODP) Site 1026, and the southern and northern sides of Mama Bare Outcrop. The composition of basement fluids is determined or estimated from analyses of pore water samples that were extracted from sediments at each of these sites, spring waters from Baby Bare, and basement fluids that vented from the open ODP Hole 1026B. Chemical trends in basement fluids along this transect show increasing alteration from south to north. A similar trend was observed along an ODP transect perpendicular to the ridge axis with increasing fluid alteration from west to east. Much of the increase in fluid alteration along the ODP transect is explained by greater water-rock exchange with increasing basement temperature to the east. In contrast, the trend along the 3.5-m.y.-old ridge is best explained by diffusive exchange with the overlying sediment. The rate of this exchange is used to constrain hydrologic properties within basaltic basement. Flow within the 3.5-m.y.-old ridge is inferred to occur from south to north and lacks significant exchange with basement fluids from the active ridge crest to the west. Thus the two flow systems are hydrologically distinct, and flow paths are likely influenced by the complex distribution of permeability in basement, the pattern of seafloor morphology, and the type and rate of sedimentation. ¿ 2000 American Geophysical Union