The objective of this study was to investigate whether melamine (MEL) is biotransformed into cyanuric acid (CYA) in male broiler chickens. The effects of the dietary addition of MEL and CYA alone and in combination on the growth performance and mortality of the chickens were also investigated. The chickens were divided into six experimental groups and were fed diets with an addition of 50 or 100 mg/kg MEL or 50 or 100 mg/kg CYA of diet, with the contaminants added separately or in combination. The control group was fed a diet without MEL or CYA. At the end of the experiment (day 40), samples of liver, kidney and breast and thigh muscles were collected from 12 birds per treatment group and were analysed for the presence of MEL and CYA. This study showed that MEL is biotransformed into CYA in broilers. Higher concentrations of MEL or CYA in the diet increased the presence of their residues in tissues, and the CYA residue concentration was several times higher in the respective tissues than the MEL residue concentration. The dietary addition of MEL and CYA did not affect the mortality rate, feed conversion ratio or body weight of the chickens.