1. The intracellular Na+ activity (aNai) of the smooth muscle cells from guinea-pig ureter has been measured using double-barrelled Na+-sensitive micro-electrodes. aiNa in modified Krebs solution at 35 degrees C was of a mean 7.4 +/- 2.9 mM (n = 32, S.D. of an observation), equivalent to a Na+ equilibrium potential (ENa) of +66.7 mV. Membrane potential (Em) was of a mean -50.8 +/- 4.6 mV. 2. Inhibition of the Na+ pump by application of ouabain or removal of external K+ (K+o) resulted in a restricted rise of aNai. The rate of rise was faster in the presence of ouabain (10(-4) M) but the stabilized aNai was not significantly different from that observed after the prolonged absence of K+o. The mean aiNa recorded after prolonged Na+ pump inhibition was 20.6 +/- 5.5 mM (n = 28), equivalent to an ENa of +39.6 mV. Neither removal of K+o after aNai had stabilized in the presence of ouabain nor application of ouabain after aNai had stabilized in K+-free solution caused a rise in aiNa, suggesting that the Na+ pump was fully inhibited by either procedure. 3. Reduction of Na+o resulted in a rapid fall in aiNa against the electrochemical gradient, both before and after Na+ pump inhibition. At each level of Na+o, aNai stabilized such that ENa remained approximately constant in either condition. Readdition of Na+o resulted in a rapid recovery of aNai. 4. Elevation of Ca2+o (at constant Na+o) caused a fall in aNai of much the same time course as that observed on reduction of Na+o, both before and after Na+ pump inhibition. The extent of the fall was dependent upon the initial aNai. Reduction of Ca2+o resulted in a rise in aNai. 5. Elevation of the external divalent cation concentration with Mn2+ or, to a lesser extent, Mg2+ reduced aiNa in the presence of a functional Na+ pump. But after prolonged exposure to ouabain or K+-free solution, elevation of Mg2+o had no effect on aiNa while application of Mn2+o caused a slow rise. These results suggest that Ca2+o affects aiNa in two ways. One is mimicked by Mg2+ and Mn2+ and is probably due to alteration of the Na+ leak. The other is a specific effect, revealed by Na+ pump inhibition. 6. It is concluded that aiNa can be maintained far from equilibrium in the absence of a functional Na+ pump. Several lines of evidence are discussed which indicate the participation of Na+-Ca2+ exchange in Na+ extrusion in this condition.