During isometric contraction, motor units are recruited depending on the order of the axonal diameter, which may be proportional to the soma size or the inverse of input resistance (IR) of motoneurons (MNs). This phenomenon is known as the size principle or rank-ordered recruitment of MNs. Because the trigeminal MNs are reported to highly express TWIK-related acid-sensitive K⁺ (TASK) 1 and TASK3 channels that are determinants of IRs and resting membrane potentials, it is possible that these channels play important roles in the rank-ordered recruitment of the masseter MNs. We first examined the mRNA levels of TASK1 and TASK3 channels by performing real-time PCR in small (< 20 μm) and large (> 40 μm) masseter MNs, which were collected using laser microdissection from tissue sections including the trigeminal motor nucleus. TASK1 mRNA appeared to be expressed 1.7 times more abundantly in large MNs compared to small MNs. TASK3 mRNA was expressed about half as much as TASK1 mRNA in large MNs while it was hardly detected in small MNs. We next examined the distributions of TASK1 and TASK3 channels in the masseter MNs using an immunohistochemical method. TASK1 and TASK3 channels were complementary distributed in soma and dendrites of masseter MNs, respectively. We also examined the possible involvement of spindle Ia inputs onto masseter MNs and TASK channels in the orderly recruitment of masseter MNs in rat slice preparations using whole-cell patch-clamp recordings. Simultaneous whole-cell current clamp recordings obtained from a small and large MN revealed that the recruitment of masseter MNs in response to stimulation of the presumed spindle Ia inputs is dependent on their soma size or IR. These results suggest that the rank-ordered recruitment of masseter MNs is dependent on the activities of TASK1 and TASK3 channels.