1. Action potentials of single plantaris motoneurons were recorded monophasically from fine ventral root filaments. A resistor was placed in shunt across the recording electrodes and its value was varied until the size of the action potentials was reduced by one-half. At this point the resistance of the filament was taken to be equal to that of the shunt, and the quotient of action-potential amplitude divided by filament resistance was proportional to the axonal action current. 2. The measurement of axonal action currents depends on the assumption that the filament-electrode system obeys Ohm's law. Tests were performed which validated this assumption. It was then shown that the axonal action currents varied as the square of conduction velocity over the range of alpha and gamma motoneurons. 3. A direct correlation was established between the critical firing levels of motoneurons and the sizes of the impulses in their axons after the recorded sizes had been normalized in accordance with the resistances of the ventral root filaments in which they were located. Since both the CFL and axonal diameter were related to impulse size, they were related to each other (Fig. 6). 4. Evidence is cited justifying the conclusion that the dimater of a motor axon is directly related to the size of its soma. Thus, it may be inferred that the critical firing level of a motoneuron is a function of its size.