The relationship between the strength of the storm-time ring current and the available density in the presumed source region, the plasma sheet, is examined for 23 storms with Dst* (minimum Dst corrected for magnetopause currents) ranging from -50 to -164 nT. A good correlation is found between the plasma-sheet density at geosynchronous orbit and the minimum Dst*. The minimum Dst* is also well correlated with the eastward interplanetary electric field measured upstream by the Wind spacecraft, in agreement with previous studies, but the dependence of Dst* on plasma-sheet density is unrelated to its dependence on the electric field. The best correlation is between the minimum Dst* and the product of the plasma-sheet density and the eastward interplanetary electric field. These results are consistent with a scenario in which the intensity of the storm-time ring current is determined by a combination of source strength (plasma-sheet density) and injection strength (interplanetary electric field). ¿ 1998 American Geophysical Union