1. Single unit impulses were recorded from the ulnar and median nerves of awake human subjects with tungsten electrodes inserted percutaneously in the upper arm.2. Forty-nine rapidly adapting mechanoreceptors with receptive fields in the glabrous skin area were studied. Thirty-nine units had small receptive fields with distinct borders (RA-receptors) while ten units had large fields with indistinct borders (PC-receptors).3. The afferent response to stimuli of varying indentation amplitude and velocity of indentation, was analysed.4. Amplitude thresholds varied from 0.05 to 1.65 mm for the RA-receptors. For the PC-receptors amplitude thresholds ranged from less than 0.05 to 1.95 mm.5. Velocity thresholds varied for the RA-receptors from 0.4 to 39.3 mm/sec, and for the PC-receptors from 0.5 to 19.6 mm/sec.6. The conduction velocities of the afferents were all in the A alpha-beta range. For the RA-receptors the conduction velocities ranged from 26 to 91 m/sec (mean = 55.3 +/- 3.4), and for PC-receptors the range was from 34 to 61 m/sec (mean = 46.9 +/- 3.6).7. The nerve impulse frequency as a function of indentation velocity was analysed for nineteen RA-receptors and four PC-receptors. A hyperbolic log tangent function of the type first introduced by Naka & Rushton (1966) in studies on S-potentials in the fish retina was found to be the best description of the stimulus-response function for sixteen RA-receptors and two PC-receptors. For the remaining units a pure logarithmic function was the best description. However, the logarithmic function may be, as found in the present study, a special case of the more general log tanh function.