1. The contractile properties of gastrocnemius muscle units and the electrophysiological properties of their innervating motoneurons were examined in anesthetized adult male Sprague-Dawley rats in situ, using conventional microelectrode techniques. 2. Muscle units (n = 70) were classified as fast (F) or slow (S) on the basis of the degree of force summation during stimulation at 25 Hz, as well as fatigue resistance, in response to current injection into innervating motoneurons. S units categorized using these criteria were also found to invariably demonstrate twitch half-relaxation times (RT1/2) >28 ms, with F units demonstrating RT1/2 <27 ms. Some overlap was present between F and S units in twitch time-to-peak tension (TPT). 3. S muscle units were innervated by motoneurons with significantly higher after hyperpolarization (AHP) times-to-peak and half-decay times, AHP amplitudes, and input resistances than F units. Motoneurons innervating S units also demonstrated slower mean axon conduction velocity than F units. 4. F units were further classified as fast fatiguing (FF), fast fatigue-resistant (FR), or fast intermediate (FI) on the basis of their fatigue resistance. Muscle unit forces were different among the unit types such that FF > FI > FR > S. Twitch TPT and RT1/2 differed such that FF < FI, FR < S. 5. No differences among FF, FI, and FR units were found for measures of AHP time course, AHP amplitude, rheobase, or input resistance. The only motoneuron property that differed among F unit types was axon conduction velocity, which was lower for FF than for FI and FR units. 6. The best relationships between muscle unit and motoneuron physiological properties were between expressions of twitch time course, which included RT, and AHP half-decay time (r = 0.73 to 0.74). S units always had AHP half-decay times >20 ms, whereas for F units, values never exceeded 19 ms. Correlations between these variables increased when only S units were considered (r = 0.86-0.97), and were weak and not significant among F units only. 7. The only other significant correlations between muscle unit and motoneuron properties >0.5 were between input resistance and expressions of twitch time course (r = 0.56-0.66). These relationships were nonexistent when type S units were not included in the analysis. 8. A sample of tibial motoneurons (n = 98) that innervated hindlimb muscles other than gastrocnemius was combined with the gastrocnemius motoneurons to examine interrelationships among motoneuron properties. Relating AHP half-decay time with input resistance (r = 0.62) and with axon conduction velocity (r = -0.52) provided the only correlation coefficients >0.5. When only ''presumptive slow'' (AHP half-decay times >20 ms) and ''presumptive fast'' (AHP half-decay times <20 ms) motoneurons were analyzed, correlations were weak and not significant. Relationships among input resistance, rheobase, and axon conduction velocity were poor, with correlation coefficients <0.41. 9. It appears that the bipartite organization of F and S muscle unit types in the rat extends to the properties of their innervating motoneurons, as is the case in cat. The distinction among motoneurons innervating F muscle units that differ in fatigue resistance is less clear. The weakness of relationships among motoneuron properties that are expressions of cell size and excitability, compared with those relationships reported for cat, suggest that the rat is not merely a scaled-down version of its feline counterpart.
