A microdialysis technique combined with a push-pull pump was applied for monitoring protein dynamics in the liver. A newly developed probe has a 0.34 x 10-mm (membrane thickness, 0.05 mm) dialysis membrane of polysulfon and can allow the passage of molecules of up to approximately a few hundred kilodaltons in molecular weight. The probe was inserted in the liver of a rat under pentobarbital anesthesia. Perfusion medium (phosphate-buffered saline) was pumped through the microdialysis probe and collected every 15 min. Effect of ischemic treatment of the protein constitution and the activity of lactate dehydrogenase (LDH) in dialysate were determined to confirm the accuracy of the present technique. The protein constitute in preischemic dialysate differed from those obtained in the serum and hepatic homogenate, showing that the dialysate reflected extracellular protein. LDH activity was high immediately after insertion of the probe, decreased constantly, and then reached a plateau of a relatively low level. When transit ischemic treatment (for 15 min) was performed by ligation of both hepatic artery and portal vein, LDH activity increased significantly, which continued for over 5 h. The concentration of albumin in the dialysate increased immediately after the ischemia. Such changes in LDH activity and albumin concentration reflected ischemic change, and the newly developed technique may be useful for the monitoring of extracellular dynamics of proteins with molecular weight less than 200 kDa.