ABSTRACT. Lidocaine hydrochloride (Lido) is widely used for analgesia in veterinary medicine; however, in humans, it has been suggested that Lido attenuates granulocyte functions, such as adhesion and reactive oxygen species (ROS) production. Thus, Lido may affect canine granulocyte function; however, there have been no reports on the effects of Lido on canine granulocyte function. Thus, we studied the effects of Lido on canine granulocyte CD11b expression and ROS production. We further studied the effects of Lido on the priming of canine granulocyte CD11b expression and ROS production by recombinant canine granulocyte macrophage colony stimulating factor (rcGM-CSF). Isolated granulocytes were incubated with 3, 30 or 300 g/ml Lido, or with Lido followed by priming with 5 ng/ml rcGM-CSF. CD11b was detected by the immune fluorescent antibody method, and the mean fluorescence intensity (MFI) was assayed by flow cytometry. ROS production was assessed by the peak time (PT) of ROS production and area under the luminol reaction curve (AUC), which represents total ROS production quantity against opsonized zymosan stimuli. Only 300 g/ml Lido (tissue level observed by regional block) significantly attenuated both the MFI of CD11b and its enhancement by rcGM-CSF. Moreover, at this concentration, the AUC and its enhancement by rcGM-CSF were significantly attenuated by Lido; in contrast, Lido did not affect PT. In conclusion, Lido suppressed granulocyte adhesion to the endothelium and antiseptic capability by suppressing CD11b expression and/or ROS production. Particular care should thus be exercised when performing regional anesthesia block using Lido.