We sought to define the contribution of the climbing fibers, one of the major inputs to Purkinje neurons, to the increase in cerebellar blood flow (BFcrb) produced by activation of the cerebellar cortex. The neurotoxin 3-acetylpyridine was used to lesion the inferior olive, the site from which the climbing fibers originate. Crus II, an area of the cerebellar cortex that receives sensory afferents from the perioral region, was activated by low-intensity stimulation of the upper lip (5-25 V, 4-16 Hz) in sham-lesioned and lesioned mice. BFcrb was recorded in crus II using a laser-Doppler flow probe. The increase in BFcrb produced by harmaline, an alkaloid that activates the climbing fibers, was abolished in lesioned mice (p>0.05 from BFcrb before harmaline; n=6/group), attesting to the effectiveness of the lesion. In sham-lesioned animals, upper lip stimulation increased BFcrb in crus II by 25±2% (25 V; 10 Hz; n=6). The rise in BFcrb was attenuated by 63±7% (25 V and 10 Hz) in lesioned mice (p<0.05; n=6). In contrast, the increase in BFcrb produced by hypercapnia was not affected (p>0.05). These data suggest that climbing fibers are responsible for a substantial portion of the increase in BFcrb produced by crus II activation. Thus, the hemodynamic response evoked by functional activation of the cerebellar cortex reflect in large part climbing fiber activity.