Characteristic biotransformation of aminopyrine in rat when administered with barbital.

After the administration of aminopyrine with or without barbital to rats, aminopyrine and its main metabolites were detected in plasma and the brain by means of gas chromatography-mass spectrometry. It was clarified that a marked increase of 4-monomethylaminoantipyrine was observed in the case of coadministration of aminopyrine and barbital, while the plasma level of aminopyrine decreased significantly compared with single administration.