The impulse responses to perpendicular sinusoidal skin displacements were recorded from 4 different types of mechanoreceptive different units innervating the glabrous skin of the human hand. The cycle responses, defined as the number of impulses evoked per sine wave cycle, were studied at a wide range of frequencies (0.5-400 Hz) and amplitudes (0.001-1mm). The rapidly adapting units (RA) were most easily excited at stimulus frequencies between 8 and 64 Hz, whereas the corresponding frequencies for the Pacinian units (PC) were above 64 Hz. However, at high stimulus amplitudes, the RA and the PC units showed quite similar response profiles within the range of frequencies tested. The sensitivities of the slowly adapting unit types (SA I and SA II) were greatest at lower frequencies. A characteristic finding for all 4 types of units was that the higher the amplitude, the lower the frequency at which the cycle response was maximal.