Abstract The effect of changes in the ionic composition of the incubation medium on the active uptake and retention of myo-inositol by cortex slices from rat kidneys has been assessed. The Na + requirement could not be satisfied by Li +, K +, Rb +, Cs +, choline + or Tris +. K + (>4 mM) was necessary for optimal uptake and retention of inositol, but could be replaced completely with Rb +, Cs + was only partly effetive in place of K +, whereas Li + and NH 4 + were ineffective. The effects on inositol uptake and leakage produced by the substitution of choline + for Na + were largely reversible, indicating that choline had not permanently damaged the slices. Neither glucose nor ATP restored the values of the two parameters examined to normal in the absence of K + or at low levels of Na +. Na + alone was not able to maintain the integrity of the system. Both Ca 2+ and Mg 2+ at optimal concentrations of 1 mM and 1–2 mM, respectively, were required, but could be replaced by Sr 2+ concentrations as low as 0.75 mM. The inhibition of inositol uptake by ouabain in Krebs-Ringer bicarbonate media also occurred, when K + was either omitted or replaced by other monovalent cations. It was partly prevented by increased concentrations of K +, Cs + or, especially, Rb +, but not by Ca 2+. The effects of cations are discussed in terms of the mechanism of active transport of non-electrolytes and cations and of the integrity of renal tubular membranes.