Abstract
The pretreatment of calves with a single dose of 10 mg kg−1 dieldrin or 21 daily doses of 10 mg kg−1 phenobarbitone increased the toxicity of diazinon as reflected by the development of more severe clinical signs and greater depression in whole blood cholinesterase activity in the pretreated calves. Induction by dieldrin or phenobarbitone of the hepatic microsomal enzyme amidopyrine-N-demethylase was also accompanied by a concurrent rise in the liver carboxyl-esterase activity.