THE metabolism and mode of action of nitriles, such as β-aminopropionitrile (BAPN), that cause osteolathyrism in rats and other animals are far from being understood. Lalich1 has shown that after injection into rats of BAPN labelled in the cyano position with carbon-14, 80–90 per cent of the radioactive material was eliminated in the urine within 20 h. About 40 per cent of the activity was in the form of unchanged BAPN and 20–30 per cent could be recovered as cyanoacetic acid. Lalich also found that cyanoacetic acid had no lathyrogenic effect when fed to young rats and suggested that it was probably a detoxication product of BAPN.