We have previously reported that traditional Japanese medicines (Kampo medicines) have an ability to reduce the peak amplitude of fast-conducting and voltage-gated Na⁺-channel blocker tetrodotoxin-sensitive compound action potential (CAP). Daikenchuto, rikkosan, kikyoto, rikkunshito and kakkonto reduced the peak amplitude of the CAP in a concentration-dependent manner; daikenchuto had a half-maximal inhibitory concentration (IC₅₀) value of 1.1 mg/ml. When compared at a concentration of 2 mg/ml, the extents of the reductions by daikenchuto, rikkosan, kikyoto, rikkunshito and kakkonto were 70, 30, 25, 15 and 12 %, respectively. Daikenchuto was the most effective in inhibiting CAPs among various kinds of the medicine. Daikenchuto is composed of three kinds of crude medicine, ginseng, Japanese pepper and processed ginger. When each of the crude medicines at 2 mg/ml was tested, Japanese pepper and processed ginger reduced CAP peak amplitudes by 70 and 30 %, respectively, while ginseng radix hardly affected CAPs. The inhibitory action of Japanese pepper had the IC₅₀ value of 0.77 mg/ml. The present study was undertaken to know whether there is an interaction among the inhibitory actions by the daikenchuto crude medicines. The experiments were performed by applying the air-gap method to the frog sciatic nerve. Ginseng radix (0.6 mg/ml), which had no effect on CAPs, unaffected the inhibitory action on CAPs of processed ginger in a range of 0.2 to 2 mg/ml, but had a tendency to enhance the inhibitions of CAPs by low (< 0.5 mg/ml) but not high (> 0.5 mg/ml) concentrations of Japanese pepper. Processed ginger (1 mg/ml) also had a tendency to increase CAP inhibition by Japanese pepper at low but not high concentrations. These results indicate that there is a positive interaction in nerve conduction inhibition among crude medicines contained in daikenchuto at low but not high concentrations.