In an effort to characterize the principal mechanism(s) underlying the methyl mercury-induced defect in protein synthesis in vivo and in vitro, we examined the dose and time related effects of methyl mercury chloride (MeHg) on a variety of cellular functions using bulk-isolated neonatal cerebellar perikarya. This cell preparation demonstrated a high specific activity for protein synthesis which was optimal between 6 and 12 days of age and declined rapidly thereafter. In vitro MeHg inhibited synthesis with an ID50 of approximately 14 microM without causing significant release of lactate dehydrogenase. The inhibition of protein synthesis occurred independently of mercurial effects on RNA synthesis, mitochondrial function, ATP content or intracellular levels of Na+ and K+. Uptake of [3H]-phenylalanine was not appreciably affected by MeHg. The accumulated evidence suggests that in this in vitro cell model, MeHg inhibition of protein synthesis occurs via a direct interaction with the protein synthetic machinery.