In order to measure in situ the kinetics of peritoneal ultrafiltration in peritoneal dialysis (PD) with long dialysate dwell, a colorimetric method using autologous hemoglobin as a marker added to the dialysis fluid was evaluated in 10 patients on PD for periods ranging from 2 weeks to 20 months. Four different dialysate formulae were successively tested in the same patient. The results expressed as normalised ultrafiltration coefficient (UFK, ml/min/1.73 m2 s.a.) showed a wide scatter among the patients. However, the dilution curves successively obtained with dialysis fluids of varying osmolar concentration in the same patient were very consistent with each other, indicating that UFK was a characteristic of the individual peritoneal membrane. The data presented suggest that UFK should be measured in each patient at initiation of PD in order to prescribe the dialysate formulae and the diffusion times fitting best his individual needs. On the other hand, repetitive measurements of UFK in the same subject would allow a useful analysis of the hydraulic permeability of the peritoneum by detecting any significant change in this parameter during long term PD.