Recent studies indicate a possible role of nitric oxide (NO) in regulating leukocyte-endothelial cell interactions, which plays a key role in the tumor immunity. The purpose of the present study is aimed to observe the tumor hemodynamics intravitally and to clarify the effect of NO on tumor microcirculation by means of a real-time confocal laser-scanning microscope using NO-reactive indicators. Visualization of localization of NO and the leukocyte behavior was made in the mesenteric microvessels of an experimental tumor model rat. Production of NO was clearly visualized along the endothelium of the tumor-free rats, but scarcely found in the newly formed tumor microvessels. A higher level of NO production was observed in a solid tumor region, where a more marked decrease in the leukocyte-endothelial cell interactions was observed. Local administration of a NO synthase (NOS) inhibitor increased leukocyte adhesion. This indicates that tumor-derived NOS creates the tumor specific microenvironment of the immature angiogenic tumor vessels, thereby modulating leukocyte behavior.