In 19 patients aged 19 to 45 years on long-term dialysis treatment during 12.6 +/- 18.2 months the concentrations of Rb and Cs were determined by atomic spectrometry in whole blood before and behind the dialyser at the beginning and end of dialysis. At the same time the concentrations of these elements were determined in the dialysing fluid. In all patients the velocity of conduction in the motor fibres in the upper and lower extremities was determined before and after dialysis. Damage to the peripheral neurons was demonstrated in the lower extremities mainly in 79% of cases. Increased velocity of motor conduction in at least one nerve related directly proportionally to the Cs concentration of the serum was demonstrated in 56-70% of the patients after one dialysis.