Menadione bisulphite increased endogenous oxygen-radical production by rat brain synaptosomes, as indicated by H2O2 generation. Increased oxygen-radical production was also demonstrated in synaptosomes prepared from menadione-treated rats and synaptosomes reoxygenated after an anoxic insult. Acetylcholine synthesis de novo was inhibited in synaptosomes incubated with menadione in vitro, in synaptosomes prepared from menadione-treated animals in vivo, and in depolarized post-anoxic synaptosomes. Intrasynaptosomal free Ca2+ was increased by menadione in vitro (50 microM), but this increase was not due to stimulation of Ca2+ entry into the nerve terminals. Acetylcholine release was stimulated by menadione in vitro, possibly as a consequence of the elevated intrasynaptosomal Ca2+ content. The Ca2+ contents of synaptosomes prepared from menadione (10 mg/kg)-treated animals in vivo and synaptosomes reoxygenated after anoxia were unchanged. In synaptosomes prepared from menadione-treated animals, acetylcholine release was no longer significantly stimulated by K+, whereas it was unchanged from control (normoxic) values in synaptosomes reoxygenated after anoxia. None of these treatments caused any measurable damage to the synaptic plasma membrane (as judged by the release of lactate dehydrogenase), or to synaptosomal phospholipases (as judged by choline release from membrane phospholipids). Synaptosomes prepared from menadione-treated rats were found to be a good model for the study of post-anoxic damage to nerve-terminal function.