On-line monitoring of dialysate conductivity is now a standard equipment (called 'Diascan') of the dialysis monitor Integra (Hospal, Italy). From the record of the dialysate conductivity at the dialyser inlet and outlet, the Diascan calculates the values of patient's plasma conductivity and of ionic dialysance which is a weighed average of the dialysances of all ions of quantitative importance in plasma and dialysate. Because there is an equivalence between the transfer characteristics of urea and electrolytes, the ionic dialysance reflects the urea clearance corrected for recirculation. Because the conductivity of a solution is related to the concentrations of the ions and thus to the effective osmolality, the plasma conductivity is a reflection of the plasma sodium concentration. The determination of ionic dialysance and plasma conductivity by the Diascan module is fully automatic and totally inexpensive, does not require any blood or dialysate sampling and therefore can be repeated every 15 or 30 min during each dialysis session. Some clinical applications of conductivity modelling are presented: (i) the repeated measurement of ionic dialysance allows the quantification of the dialysis dose actually delivered to the patient from the beginning of the session; (ii) the measurement of ionic dialysance with blood lines in normal and reversed positions permits the easy estimation of the blood flow rate in the vascular access of the haemodialysed patient; (iii) the on-line monitoring of ionic dialysance allows the development of new methods of haemodialysis with simultaneous infusion of ions; (iv) the on-line monitoring of ionic dialysance and patient's plasma conductivity facilitates the automatic optimization of the dialysate conductivity for each individual patient.