Fluorescein-labelled sulodexide pharmacokinetics and urinary excretion were studied in the rat after intravenous administration. The compound showed a pharmacokinetic pattern similar to that of heparin and the plasmatic levels were linearly correlated with anti-factor Xa activity and logarithmically with lipoprotein lipase activation. The compound is well eliminated by urinary route; the urinary recovery was 50% in 24 h and 67% 48 h after administration. The results obtained indicate that fluorescein labelling of glycosaminoglycans may be a valid tool for pharmacokinetics and distribution studies in the laboratory animals.