The activity of single motoneurones of m. flexor carpi ulnaris (FCU) was investigated by recording their motor unit (MU) action potentials during weak and moderate voluntary muscle contractions. The MU firing rate range was 4.5-15 imp/s. The excitability of motoneurones was tested with a number of single stimuli eliciting a monosynaptic H-reflex of low amplitude. Two different indices were defined which relate to motoneuronal excitability: the response index -the ratio of the number of responses of a motoneurone to the total number of stimuli, and the response time-the time after the last background MU discharge at which motoneurone is ready to respond to the excitatory volley. Both the response index and the response time were determined for single motoneurones at different levels of background activity. In the lower range of firing rates, the response index for all motoneurones decreased when increasing the firing rate, but it remained constant in the higher rate range. This kind of response seems to be a typical motoneuronal response to the stimulation with single stimuli. The data on the response time were used to study the excitability of the same single motoneurones to computer simulated repetitive stimulation (stimulation rate 40-100imp/s). In this case, the excitability of each motoneurone was determined as an increment of its firing rate in response to the stimulation. For the lower firing rate range, the excitability for all motoneurones also decreased when the firing rates increased whereas a variety of slopes was obtained in the higher rate range. The results of our experiments and their computer simulation allow us to suggest a possible interrelation between the excitability of firing motoneurones and their interspike membrane potential trajectories.