A rapid reconfiguration of the dusk side magnetosphere was observed near the magnetic equator by particle and field experimens aboard Explorer 45 at 1715 UT on June 17, 1972. Following the onset of an 800-&ggr; negative bay in the auroral zone, Explorer 45, inbound at 1700 MLT and L~5, observed a sharp dropout of outer zone electrons (35-560 keV). Simultaneously, the plasma density dropped from a level typical of the plasmasphere to one typical of the ion trough, and plasma sheet proton fluxes (3.5-138.5 keV) increased, producing 1 100-&ggr; depression in B, in a shift to a taillike configuration. Outer zone electrons with pitch angles near 45¿ returned with delay times consistent with injection near midnight at the time of dropout and eastward drift to the dusk meridian. The equatorial electrons were not observed, since shell splitting apparently placed them on pseudotrapped orbits. Approximately 45 min after the reconfiguration, the satellite reentered the plasmasphere, and the electron fluxes returned to previous levels. The injected protons extended well into the plasmasphere. In the next orbit, approximately 6 hours later, a region of isolated plasma was detected near apogee at 1400 MLT. There is therefore circumstantial evidence that the type of magnetospheric reconfiguration observed here is responsible for the detachment of regions of cold plasma from the plasmasphere.