Various concentrations of acetylcholine (ACh) were detected in samples of bovine, goat, horse, porcine, rat and sheep blood and plasma using a specific, sensitive radioimmunoassay. The ACh levels in whole blood in bovine and horse samples were about 40- and ten-fold higher, respectively, than in humans, but levels comparable to those in humans were measured in porcine samples. Goat, rat and sheep samples had lower whole blood ACh concentrations than those of humans. When plasma samples were assayed, the ACh contents of bovine and porcine plasma were found to be about two- to five-fold those of human. On the other hand, levels in horse, goat, rat and sheep samples were much lower than in humans. The ratio of the ACh contents of plasma to whole blood was high in porcine and rat samples, indicating that porcine and rat blood ACh is distributed mostly in the plasma, while in the other species tested most of the ACh is present in the blood cells. These results demonstrate that variable levels of ACh are present in the blood of different species, and that the distribution of ACh in the blood constituents varies according to species.