In the course of our study on the drug-processing of the seeds of Strychnos nux-vomica L. (Loganiaceae), the alkaloid composition of the heat-treated seeds of S. nux-vomica was compared to that of the untreated seeds. On heat treatment, the contents of the major alkaloids such as strychnine and brucine declined significantly with increases in the amounts of isostrychnine, isobrucine, strychnine N-oxide and brucine N-oxide. The cleavage of an ether linkage and the occurrence of N-oxidation were demonstrated by heat treatment of authentic strychnine and brucine.