Core experiments with radioactive tracers are reported in which an artificialradioactive material, radiovanadium, was used in aqueous solution in a study ofthe mobility of connate water. Sandstone cores saturated with active water werefirst oil-flooded to simulate virgin sand conditions in which the pores arefilled with oil and connate water. They were then subjected to flooding withinactive water. Measurements were made of the activity of various portions ofthe effluent as well as the activity of the core after the flooding process wascompleted. In additional experiments the distribution of activity along thecore was measured during the flooding process. In still other experimentsdirect flooding tests were made on cores fully saturated with active water. Inall cases it was found that after injecting into the core a volume of inactivewater equal to a pore volume, the effluent contained more than 80 per cent ofthe active content originally in the core. It is felt that these results implya high degree of mobility of the connate water when subjected to flooding, although at least a partial contribution of the effect of tracer transferwithin the core cannot be definitely excluded. The implications of this resultwith respect to water-flooding, core analysis and electric logging arediscussed. A better approach to determining the connate water saturation ofoil-producing rocks, by measuring the fluid content of core samples at thesurface, is by the use of crude oil or oil-base muds as the coring fluid. Introduction Although water flooding is of considerable importance in the recovery of oil, comparatively little is known with certainty about the role played by theconnate water during the operations. Two possibilities exist. Either theconnate water is retained in the formation while the flood water sweeps past, or it is pushed along to a greater or lesser degree by the flood. Thus far noexplicit recognition in terms of quantitative studies appears to have beengiven to the displacement of connate waters by the intruding extraneous waters, although such a possibility might be important in determining the efficiency ofthe oil -displacement process in water-flooding operations, especially inargillaceous rocks. On the other hand, it is a common observation that thefirst water produced with the oil as a result of water-flooding is definitelysaline. Similar questions regarding the mobility of the interstitial waterarise in the quantitative description of the advance of edge-water drives. T.P. 2054