Threshold electrotonus (TE) is a new tool for investigating axonal function noninvasively in vivo. To increase its potential clinical value, we developed a rat model of TE, and examined the effects of maturation and pharmacological intervention. We recorded TE in 92 male rats (body weight 90-650 g) by stimulating the motor nerve in the tail, and applying 100-ms conditioning currents. Motor conduction velocities increased up to a body weight of 330 g, and remained constant thereafter. TE in mature rats was similar to that in humans, and two parameters were analyzed: TEd 10-20 or the mean threshold reduction 10-20 ms after the onset of the depolarizing conditioning current at 40% of threshold intensity; and TEh 10-20 or the corresponding threshold decrease on hyperpolarization. Like latency, the absolute value of TEh 10-20 decreased up to 330 g, and then stabilized thereafter, probably reflecting the progressive increase in the axonal diameter and relative reduction in internodal impedance. In contrast, TEd 10-20 gradually decreased up to 330 g, and then jumped to a higher level, which was maintained for animals of >400 g. 4-Aminopyridine, a blocker of fast potassium channels, selectively increased TEd 10-20 only in the immature or young (<330 g) rats. This suggests that, in the mature animals, fast potassium channels become sequestrated from the nodal membrane and not activated in response to nodal depolarization. These findings indicate that mature rats (>400 g) may provide a useful experimental model for interpreting abnormal TE responses in humans, and provide evidence for nonlinear maturation of potassium channel function in myelinated axons.