Plasma flows during plasma sheet thinnings at the expansive phase of magnetospheric substorms are extensively examined on the basis of Imp 6 data obtained by the Lepedea from the University of Iowa and by Geiger counters from the University of California, Berkeley. The Imp 6 satellite surveyed the plasma sheet within a geocentric distance of ~32 RE. During the period from September 1971 to December 1972, 74 plasma sheet thinnings are identified. It is found that plasma flows during thinnings are directed most frequently sunward, not antisunward, in the midnight sector (‖YSM‖?10 RE), within XSM?-30 RE. Thus the plasma flow observations do not support the conjecture of a magnetic neutral line formation in the near-earth plasma sheet, namely, within XSM?-18 RE, at the substorm onset for at least most magnetospheric substorms. Several modifications of the existing neutral line model are considered, but none seems to account for the most common behavior of the plasma flow (i.e., sunward within X?-30 RE) and the Bz component near the midplane of the plasma sheet (i.e., positive within X?-60 RE) that are observed during the substorm expansive phase. It is suggested that the plasma sheet behavior during its thinning is consistent with a deflation of the plasma sheet in which plasma sheet particles are drained earthward and the energy source for the substorm expansive phase is primarily the energy associated with the sunward plasma flow from the deflation (thinning) of the plasma sheet.