While applying the Fast Fourier Transform to the instantaneous frequency responses to sinusoidal indentations of the cutaneous mechanoreceptor afferent units in frogs, we examined quantitatively the dynamic responses of these units. In two kinds of slowly adapting (SA) units, i.e., frog type I and frog type II units, and in rapidly adapting (RA) type I units, the instantaneous frequency responses could be reconstructed by summation of the DC component and the major 3-5 harmonics in the power spectra. In both types of SA units, the fundamental wave was the largest in power of the spectrum, but in the RA units, the 2nd harmonic was the largest. In SA units, the phase of the fundamental wave advanced by 20-55 degrees relative to the sinusoidal stimulation, but the phase of the 2nd harmonic of the RA units advanced by ca. 90 degrees. The magnitude of each component in the power spectra, especially the fundamental wave of the two SA units and the 2nd harmonics of the RA unit, increased with an increase in stimulus amplitude and frequency. The phases of the harmonics in both SA and RA types were fairly constant over varying amplitudes and frequencies of sinusoidal stimulation. The present findings indicate that both the frog type I and type II cutaneous mechanoreceptor afferent units detect both indentation magnitude and positive velocity, and that the RA type I cutaneous mechanoreceptor afferent units detect the stimulus velocity.