To evaluate the usefulness of the tissue-microdialysis technique in humans, the glucose concentration in the intercellular water space was measured in the abdominal subcutaneous region in healthy subjects. A 30 X 0.3 mm dialysis fiber with a 3,000 MW cutoff was used. The dialysis catheter was calibrated in vivo by perfusing it with isotonic saline and four to five different glucose concentrations (0-5 mM). The perfusate was collected in 6-min fractions. Regression analysis of the results of the calibration yielded the perfusate glucose concentration, which was in equilibrium with the surrounding tissue. Validation experiments showed that this value could be precisely measured and represented the intercellular glucose concentration. The recovery of glucose in the dialysate (dialysate glucose concentration/medium) during the calibrations in vivo was only approximately one-half of that in vitro (recovery factors 0.28 vs. 0.44, respectively). Under steady-state conditions, the intercellular glucose concentration was similar to the glucose levels in the cubital vein. It is concluded that this microdialysis technique is a useful tool allowing measurements of metabolically active substances in the intercellular water space in vivo provided that the calibrations are properly performed.