1. The stretch reflexes of the human abductor digiti minimi (ADM) and biceps brachii muscles were compared using small-amplitude sinusoidal stretching at 10-50 Hz and recording the surface EMG. The stimulus was applied either to the relevant proximal phalanx or to the biceps tendon while the muscle studied was contracting; the same amplitude was used for all frequencies (range 0.5-2 mm for ADM, 0.1-1 mm for biceps). 2. As the frequency increased, the response of ADM decreased while that of biceps increased. Neither muscle showed a minimum at 20-25 Hz, as previously found for wrist muscles and attributed to an interaction between short- and long-latency components of the reflex. 3. For both muscles, the phase of the response lagged behind the stimulus by an amount which increased approximately linearly with frequency, without the gross inflexion found for wrist muscles. Such linearity would be found for a system dominated by a fixed time delay; its value sets the slope. The slope for biceps was half that for ADM. The values of reflex delay calculated from the slope of the phase plots agreed reasonably with the absolute latencies of the responses evoked by tap or ramp stimulation. Part of the difference between the muscles was due to differences in peripheral conduction time, since ADM lies more distally. Most of it, however, was due to different reflexes being involved, with biceps being predominantly controlled by short-latency pathways and ADM by long-latency pathways. 4. For both muscles, the phase lag at any given frequency was less than that expected from the reflex latency, determined from the slope of the phase plot. Thus, sensory transduction and central transmission had produced a phase advance in the reflex. The 'neural phase advance' of biceps was appreciably larger than that of ADM, and more than would be expected from the behaviour of its spindle afferents. The excess is suggested to be due to the action of Renshaw inhibition, which ADM may lack. 5. The results were substantiated by recording from single motor units in biceps. Stretching at the present amplitudes had rather little effect on the overall rhythmic behaviour, as shown by interspike interval histograms. However, cycle histograms showed that the discharge was modulated reasonably sinusoidally by the stretching, whatever its frequency (i.e. the probability of the occurrence of a spike varied over the cycle). Cyclic changes were also found in autocorrelograms and amplitude spectra of the spike trains.(ABSTRACT TRUNCATED AT 400 WORDS)