1. The relationship between the motor unit discharge pattern (rate and variability) and synchronization of motor unit pairs was studied in the first dorsal interosseus muscle of human subjects. In separate trials of up to 4 min duration, subjects voluntarily controlled the mean discharge rate of an identified motor unit at one of several prescribed rates (range 7.5-17.5 Hz). 2. The effect of discharge rate on the synchronous peak in the cross-correlogram was examined in eighty motor unit pairs from six subjects. Five commonly used synchronization indices were used to quantify synchrony in the cross-correlograms constructed from different discharge-rate trials. For each synchronization index, the apparent magnitude of synchrony increased at lower motor unit discharge rates. The synchronization indices were not equally sensitive to discharge rate; increases in the different indices ranged from 72 to 494% between the highest and lowest discharge rates. 3. A model of the membrane potential trajectory underlying rhythmic motoneuron discharge was used to explain the observed increase in the magnitude of the synchronization indices at lower discharge rates. The essential feature of this model is that the probability of a common-input EPSP causing a synchronous discharge in two motoneurons is independent of discharge rate. This means that the number of synchronous action potentials in excess of chance in any trial depends on the properties of the common-input EPSPs and the duration of the trial, but is not related to motor unit discharge rates. The model also demonstrated that when the excess synchronous counts are normalized to motor unit discharge rate, or baseline counts in the histogram (as in the conventional synchronization indices), the magnitude of the index increases when the motor unit discharge rates are low. 4. The strength of common input to motoneurons could be misinterpreted if conventional synchronization indices are used because of discharge-rate effects. The model was used to derive an index of the strength of common input to motoneurons (CIS) that was independent of motor unit discharge rate. CIS is the frequency of synchronous action potentials in the motor unit pair in excess of those expected due to chance (calculated during periods of tonic discharge in both units). The mean CIS in first dorsal interosseus motor unit pairs ranged from 0.052 to 1.005 extra synchronous action potentials per second across subjects. 5. Discharge variability was correlated with each of the synchronization indices and the CIS.(ABSTRACT TRUNCATED AT 400 WORDS)