The causes of central nervous system (CNS) dysfunction in uremia are not well known and are not completely reversed by dialysis. This problem was investigated in synaptosomes, which are membrane vesicles from synaptic junctions in the brain. We measured Na uptake under conditions of control, veratridine stimulation, and tetrodotoxin inhibition, in synaptosomes from normal and acutely uremic (blood urea nitrogen, 250 mg/dl) rats. In the control state, maximal Na uptake was 2.2 +/- 0.2 and 1.9 +/- 0.3 nmol/mg of protein in normal and uremic synaptosomes, respectively. With veratridine stimulation, Na uptake was increased by 1.9 and 3.6 nmol/mg of protein in normal vs. uremic rats (P less than 0.001). The increased veratridine-stimulated Na uptake observed in uremia could be due either to increased membrane permeability to Na or decrease in the Na-K ATPase pump activity. To investigate this, we studied the Na-K ATPase pump function by evaluating uptake of K (using rubidium as a tracer), uptake of Na during ATP stimulation, and inhibition of Rb and Na uptake by ouabain. In uremic rats both Rb uptake and ATP-stimulated Na uptake were significantly less than in normals (P less than 0.005). This suggests a defect in the Na-K ATPase pump. Membrane permeability for Na was then evaluated both by measuring initial Na uptake, and with addition of valinomycin. No change in Na uptake pattern was observed with valinomycin, and initial Na uptake was not significantly different in normal versus uremic synaptosomes. These data show that (a) in uremic rats veratridine-stimulated Na accumulation is significantly greater than normal; (b) the increased Na accumulation observed in uremia appears to be due to alterations in Na-K ATPase pump activity; and (c) the altered Na accumulation observed is probably not due to a uremic environment, but may be secondary to a physiologic alteration in synaptosomal function due to the uremic state. These abnormalities may affect neurotransmission and may be associated with the CNS alterations observed in uremia.