Choline-free plasma (CFP) was induced in rats by intravenous (IV) injection of 56.0 x 10(2) units kg-1 of choline oxidase (ChO) which completely metabolized the free Ch circulating in the plasma for at least 15.0 h and caused subsequent significant decrease in the concentration of free Ch in the three brain regions examined, the striatum, hippocampus, and cortex. However, the treatment did not affect concentrations of acetylcholine (ACh) in these regions. By contrast, intraperitoneal (IP) injection of 1.0 mmol kg-1 Ch chloride resulted in a maximum concentration of free Ch in plasma in 5 min, after which tissue Ch in all regions examined increased (p < 0.001). Concomitant increases were observed in cortical and hippocampal ACh (p < 0.05) 20 min after the injection. It is thus suggested that the brain may possess compensative mechanisms to prevent the supply of free Ch from circulating to the brain during synthesis of ACh in the brain. It is also suggested that the CFP rat would be a useful and readily available animal model for future study.