Studies on synaptosome mitochondrial respiration are complicated by "free" mitochondria. Veratridine stimulation of synaptosomal respiration was due to increased Na+ cycling at the synaptosome membrane associated with increased oxidative phosphorylation of intraterminal ADP and was inhibited by oligomycin, ouabain or Na+ free medium. Atractylate or carboxyatractyloside failed to block veratridine-stimulated respiration but inhibited exogenous-ADP-stimulated respiration. Protein synthesis in the synaptosome fraction was inhibited by oligomycin, valinomycin or 2,4-dinitrophenol but was unaffected by excess atractylate. No change in synaptosomal adenine nucleotide content was found in the presence of atractylate, although a significant decrease in the [ATP]/[ADP] was found with oligomycin, veratridine or valinomycin. These findings show that atractylate does not modify intraterminal mitochondrial energy transduction and indirectly suggest an impermeability of the synaptosome membrane to atractylate.