The interaction between the veterinary drug sodium sulphadimidine and nitrite has been studied under acid conditions and the formation of 1,3- di-(4-[ N-(4,6-dimethyl-2-pyrimidinyl)sulphamoylphenyl)triazene (DDPSPT) was demonstrated. This compound was not mutagenic when tested on Salmonella typhimurium and Drosophila melanogaster. In addition to the formation of DDPSPT, desaminosulphadimidine was identified as a minor reaction product.