The possibility of determining the concentration of fluorescein-labelled tracers in the interstitial fluid after intravascular injection has been investigated. Fluorescein-labelled dextran (FITC-dx) of M w 2300 was used as test molecule. Experiments were also carried out to investigate the in vivo protein binding properties of the tracer. The obtained results indicate that FITC-dx does not become bound to protein in in vivo conditions. Interstitial fluid samples from the subcutaneous tissue of rabbits were collected with a micropuncture technique using a glass pipette of 8–12 μm of tip diameter. The volume of fluid obtained was of 2–5 nl. The amount of fluorescence in the plasma and interstitial fluid samples was determined in a microscope fluorimeter. The interstitial fluid-plasma concentration ratio ( C if C p ) of FITC-dx was followed up to 4 hr after injection and reached a value of 1.0 within 2 hr. Comparison between the calculated interstitial fluid concentration of FITC-dx at 50 and 200 min and the measured values gave good agreement. The overall findings of the present investigation suggest that the fluid collected is true interstitial fluid, indicating that the technique is useful for measurements of fluorescein-labelled tracers in interstitial fluid in studies of capillary permeability.