The in vivo rates of protein synthesis in the peripheral nervous tissues of methylmercury-treated rats (10 mg/kg/day, for 7 days) have been estimated with improved methods by the injection of a large amount of [1-14C]valine of low specific activity. Protein synthesis activity in the dorsal root ganglia was inhibited to the extent of 60% of the control as early as day 5 and this continued to the symptomatic period (day 15) on which crossing of hind limbs, a typical sign of organomercurial poisoning, was observed in the animals. The sciatic nerves and dorsal roots increased protein synthesis by 56% at the symptomatic period. These increases in protein synthesis may be due to the stimulation of reactivity of Schwann's cells. On the contrary, the protein synthesis in the ventral roots showed a gradual decrease as the intoxication proceeded and decreased to 73% of the control at the symptomatic period, being similar to the case of brain. The double-labeling studies with sodium dodecyl sulfate/polyacrylamide gel electrophoresis exhibited that methylmercury inhibited the synthesis of the dorsal root ganglion proteins non-uniformly in various apparent molecular sizes, especially on day 10.