The metabolic inhibitor iodoacetate (IAA) at a concentration of 10−3 M caused Na and Ca gain and K and Mg loss from rat uteri. Ethacrynic acid (ETCA) also at 10−3 M has similar effects except on Mg. The residual cation contents (bound cation) of the washed particulates from homogenized normal or inhibited rat uteri were similar. Both types of particulates contained a larger proportion of tissue Ca and Mg than of tissue Na or K. Particulate material from IAA-inhibited tissues bound less K and more Ca than controls and exchanged more K for Na than controls. In unwashed particulate material there was more Na and Ca and less K and Mg in IAA-treated compared with similar material from controls. Though they were in the right direction the above differences did not account for the altered ion contents of metabolically inhibited intact cells. Our studies of passive ion binding did reveal at least three types of binding sites: those from which cations could be removed by washing, those from which they could be removed by ion exchange (Na, K, Ca, or Mg) or EDTA (Ca and Mg), and those from which they could not be removed by either of these procedures.