The present study examined the effects of E. coli infection on the expression levels of P-gp and CYP3A (cytochrome P450, family 3, subfamily A) in the liver, kidney and small intestine to clarify whether different expression levels of P-gp and CYP3A affect intestinal absorption, biliary secretion and kidney excretion of enrofloxacin in broilers. This evidence concerns the gene CYP3A4 and escherichia coli infection.