1. Electrical and contractile (isometric) properties were studied for sixty-six motoneurone-muscle unit combinations from rat medial gastrocnemius (MG). The animals were anaesthetized with pentobarbitone. 2. The muscle units were classified into S (slow) and F (fast) on the basis of measurements of speed and fatigue resistance: the 'slow' category comprised units with an initial twitch contraction time exceeding those found among fatigue-sensitive units (border value 20 ms). 3. Twitch speed was assessed by three different measures: (i) contraction time (time to peak, range 11.4-28.0 ms), (ii) half-relaxation time (8.4-56.5 ms), and (iii) total twitch duration (34-116 ms). All three measures were mutually highly correlated and their respective values showed a continuous and unimodal distribution across the unit population. 4. The motoneurones were investigated with regard to their time course and amplitude of post-spike after-hyperpolarization (AHP; range of total durations 30-116 ms, amplitudes 0.9-8.0 mV), rheobase (0.8-17.1 nA), input resistance (0.8-5.1 M omega) and axonal conduction velocity (33-85 m/s). 5. Motoneurones of slow-twitch muscle units (type S) had, on average, a significantly slower time course of AHP, a smaller rheobase, a higher input resistance and more slowly conducting axons than those innervating fast-twitch muscle units. 6. Across the whole neuronal sample, input conductance (reciprocal of input resistance) correlated well with rheobase (r = 0.74). However, the differences in rheobase did not seem to be caused exclusively by the associated differences in input conductance. 7. Throughout the sampled population, the relative slowness of AHP showed a continuous and highly significant correlation with the relative slowness of the corresponding unit twitch. The absolute duration of AHP was close to that of the twitch. In the Discussion it is argued that this 'speed match' between motoneurone and muscle unit would help ensure that barely recruited motoneurones start firing at a frequency that is optimally suited for the subsequent rate gradation of force. 8. AHP amplitude was, on average, significantly smaller for fast-twitch than for slow-twitch motoneurones. Calculations indicated that these differences were almost completely caused by the associated differences in input resistance; the computed value for the conductance change underlying the AHP was nearly the same for fast- and slow-twitch motoneurones. 9. A simple neurone model was used to calculate the consequences of the differences in AHP amplitude and duration for repetitive discharge properties of fast and slow cell categories.(ABSTRACT TRUNCATED AT 400 WORDS)