Samples of water and associated gases were collected from the Cajon Pass well using downhole samplers, and from the pipe stands at the completion of drill stem tests. The fluids were recovered from fracture systems in granitic rocks from two test intervals located at 1,829 to 1,905 m and 1,829 to 2,115 m. The intervals were initially cleaned by circulating into the hole fresh water and water spiked with fluorescein. Results of ΔD and Δ18O values of formation water from this well and groundwater wells in the area indicate a meteoric origin, but recharge areas probably had higher elevations than that of the well or the recharge occurred in Pleistocene time when the mean annual temperatures were 3--6 ¿C lower than at present. The large differences in the ΔD and Δ18O values of formation water indicate isolation of water within relatively proximal fracture systems. The isotope values of formation water plot on the Global Meteoric Water Line implying little water-rock interaction.