BACKGROUND: The purpose of this study was to investigate whether the effects of mivacurium on muscular relaxation were similar by the duration of more than 2 weeks after the injury of lower motor neurons in rabbits. METHODS: The animals were divided into five groups. The control group was without lower motor neuron injury. In the experimental groups, the lower motor neuron injury was made by denervating with 75 - 80% lesion on the common peroneal nerve to the right anterior tibialis muscle. The experimental groups were subdivided as 1, 2, 3 and 4 week groups (referred to ad the 1 wk, 2, 3 and 4 wks group) according to the durations of the denervation of common peroneal nerve, respectively. The dose-response relationship of mivacurium on the muscle twitches induced by TOF (train of four) stimulation (supramaximal stimulus of 0.2 ms duration, square-wave pulses, 2 Hz rate, repeated every 10 seconds) was studied by calculating ED50 and ED95 in the anterior tibialis muscles and compared between all groups. After recording the muscle twitches, microscopic findings were observed. RESULTS: The effective dose for 95% twitch depression (ED95) of mivacurium at 1week after denervation was significantly higher than that of the control group (P <0.05), but the ED95 of 2, 3 and 4wks groups were not significantly different from that of the control group. However, the ED95 of 3 and 4wks group were inclined to be lower than that of the control and significantly lower than 1wk group (P < 0.05). There was no significant difference in the effective dose for 50% twitch depression (ED50) of mivacurium in all groups. The size of the anterior tibialis muscle was significantly decreased at 4weeks after the lower motor neuron injury (P <0.05), but the number of its sarcoplasmic nuclei was increased, according to the duration after the denervation. CONCLUSIONS: Our results therefore suggest that neuromuscular response of denervated anterior tibial muscle was resistant to intravenous mivacurium in early periods of 1 or 2 weeks but sensitive 4 weeks after the lower motor neuron injury.