1. The discharges of pairs of individual motor units were recorded from intrinsic hand muscles in man. Single motor unit recordings were obtained either when both members of the motor unit pair were within first dorsal interosseous muscle (1DI:1DI recordings) or where one motor unit was within 1DI and the other in second dorsal interosseous muscle (1DI:2DI recordings). The pairs of motor unit spike trains were cross-correlated in the time domain and the results compared to those of coherence analysis performed on the same spike train data. Central peaks were present in the cross-intensity functions, indicating the presence of common synaptic input to the motoneurone pair. Coherence analysis of these data indicated significant association between motor unit firing in the frequency ranges 1-12 and 16-32 Hz. 2. Analysis of sequential non-overlapping segments of data recorded from individual motor unit pairs, demonstrated that both the central cross-intensity peak and coherence in the frequency bands 1-12 and 16-32 Hz were consistent features throughout a long recording. In these sequential recordings, the size of the central cross-intensity peak and the maximal value of coherence in the frequency band 16-32 Hz covaried from segment to segment. Analysis of the entire population of motor unit pairs confirmed a positive relationship between the magnitude of peak coherence and the size of the central cross-intensity peak. 3. Voluntary sinusoidal co-modulation of the firing rates of pairs of individual motor units recorded from within 1DI was found to produce significant values of coherence corresponding to the frequency of the common modulation. However, firing rate co-modulation was not found to affect either the size of the central cross-intensity peak or the maximum value of coherence in the frequency band 16-32 Hz. 4. Pairs of single motor units were recorded from within 1DI and biceps brachii muscles of healthy subjects. The number and size of the central cross-intensity peaks and coherence peaks detected were compared for the two muscles. The incidence and size of central cross-intensity peaks and the incidence and magnitude of 16-32 Hz coherence peaks were both found to be greater for 1DI recordings when compared to biceps brachii recordings. 5. Single motor unit recordings were made from the intrinsic hand muscles of a patient with severe peripheral deafferentation. Time- and frequency-domain analysis of these recordings revealed cross-intensity peaks and frequency bands of coherence similar to those seen in healthy subjects. 6. Cross-correlation and coherence analysis was performed between 1DI:1DI motor unit pairs from the affected and unaffected hands of eleven stroke patients. In comparison to recordings from sixteen healthy subjects, recordings from the affected 1DI of stroke patients showed a reduction in the incidence and size of central cross-intensity peaks. The incidence and size of significant peaks in the coherence spectra were also reduced. The difference between the healthy subjects and the stroke patients was most marked for the frequency range 16-32 Hz. 7. Results using a conductance-based model of the motoneurone suggest that common motoneurone EPSPs may exert a low-pass filtering effect on the coherence detectable between pairs of motor unit spike trains. EPSPs do not contribute discrete frequency components to motor unit coherence. Coherence spectra in the frequency ranges 1-12 and 16-32 Hz are likely, therefore, to reflect periodicities in the firing of common motoneurone inputs. 8. It is concluded that significant values of coherence may be found in the frequency ranges 1-12 and 16-32 Hz. The higher frequency range of coherence may reflect periodic activity in the presynaptic inputs responsible for the production of central peaks in the cross-intensity function. The lower frequency range may reflect periodic activity in other types of input common to pairs of motoneurones. Evidence from recordings made from healthy subjects and from subjects with central and peripheral neurological lesions suggests that the presynaptic inputs responsible for central cross-intensity peaks and 16-32 Hz coherence arise from central motor rather than peripheral afferent neural pathways.