Simultaneous measurements of the interplanetary magnetic field (IMF) by Explorer 35 and of the magnetotail plasma by Vela satellites are examined for the period August--October 1970. Nineteen cases are found in which the IMF turns southward more than 40¿ (‖Δϑ‖>40¿) in less than 10 min and Vela satellite, operating in a high time resolution mode, is in the plasma sheet. Twelve of these cases show a large decrease of plasma pressure, or the so-called plasma sheet thinning, during the subsequent period of southward IMF. Polar magnetic substorms occur at the earth in conjunction with these twelve plasma sheet thinnings. On the other hand, seven cases show only slight plasma sheet thinnings, and there was no indication of substorms at the standard auroral zone stations. Thus it is likely that the southward IMF alone does not reduce significantly plasma pressure in the plasma sheet and that a large decrease of plasma pressure is associated with a substorm.