Background:Few studies have focused on peripheral nerve conduction during exposure to microgravity.The-6° head-down tilt (HDT) comprises an experimental model used to simulate the space flight environment.This study investigated nerve conduction characteristics of rhesus monkeys before and after prolonged exposure to HDT.Methods:Six rhesus monkeys (3-4 years old) were tilted backward 6° from the horizontal.Nerve conduction studies (NCSs) were performed on the median,ulnar,tibial,and fibular motor nerves.Analysis of variance with a randomized block design was conducted to compare the differences in the NCS before and 7,21,and 42 days after the-6° HDT.Results:The proximal amplitude of the CMAP of the median nerve was significantly decreased at 21 and 42 days of HDT compared with the amplitude before HDT (4.38 ± 2.83 vs.8.40 ± 2.66 mV,F =4.85,P =0.013 and 3.30 ± 2.70 vs.8.40 ± 2.66 mV,F =5.93,P =0.004,respectively).The distal amplitude of the CMAP of the median nerve was significantly decreased at 7,21,and 42 days of HDT compared with the amplitude before HDT (7.28 ± 1.27 vs.10.25 ± 3.40 mV,F =4.03,P =0.039;5.05 ± 2.01 vs.10.25 ± 3.40 mV,F =6.25,P =0.04;and 3.95 ± 2.79 vs.10.25 ± 3.40 mV,F =7.35,P =0.01;respectively).The proximal amplitude of the CMAP of the tibial nerve was significantly decreased at 42 days of HDT compared with the amplitude before HDT (6.14 ± 1.94 vs.11.87 ± 3.19 mV,F =5.02,P =0.039).Conclusions:This study demonstrates that the compound muscle action potential amplitudes of nerves are decreased under simulated microgravity in rhesus monkeys.Moreover,rhesus monkeys exposed to HDT might be served as an experimental model for the study of NCS under microgravity.