The purpose of this investigation was to clarify whether the light exposure of fluorescently marked leukocytes and/or plasma influences the leukocyte behavior (rolling and firm adhesion). Anaesthetized Balb/c mice were laparotomized to expose the mesentery of the terminal ileum. Animals were randomly assigned to 5 experimental groups. One group was studied in trans-illumination mode, the other four groups received iv-bolus injections of either FITC-dextran (FITC-dx) 150 kD, rhodamine 6 G (rh6G), a combination of FITC-dx and rh6G, or acridine orange, respectively, and were then studied in epi-illumination mode. In each animal, eight vessels (6 venules, 2 arterioles) were exposed five times for 60 sec. to continuous light of the appropriate excitation wavelength with a 20-min. interval between exposures. The vessel diameters and leukocyte-endothelial cell interactions were quantified using intravital fluorescence microscopy. The diameters of arterioles and postcapillary venules remained unchanged in all groups studied. Rolling and adherent leukocytes were observed in postcapillary venules only and there were no significant differences between all groups. Under these conditions, the exposure of fluorescently labeled leukocytes and/or plasma to standard light levels for up to 300 seconds has no significant impact on leukocyte-endothelial cell interactions in mesenteric postcapillary venules.