1. The objective of this study was to determine the effects of the gluconeogenesis inhibitor metformin on 21-d old chickens. The following parameters were measured in the liver and kidney: plasma glucose, plasma mannose, enzyme activities and mRNA expression levels of glucose-6-phosphatase (G6Pase), and phosphoenolpyruvate carboxykinase (PEPCK). 2. Chickens were divided into two groups, and received either metformin (300 mg/kg body weight) or water. Plasma glucose and mannose concentrations were analysed by high performance liquid chromatography (HPLC). G6Pase and PEPCK activities were determined by glucose 6-phosphate and malic acid substrate methods, respectively. The expression levels of mRNA were determined by real-time PCR. 3. Plasma glucose and mannose reached their lowest concentrations 1 h after metformin administration. At 0 center dot 5 h-1 h after metformin administration, the enzyme activities and mRNA expression levels of G6Pase and PEPCK reached their lowest point in the kidney and their highest point in the liver. The decrease observed in the kidney may have been associated with reductions in both plasma glucose and mannose concentrations. 4. In conclusion, the effect of metformin on the kidney of chickens is similar to its effect in mammals. In contrast, no suppression of enzyme activity or mRNA expression was observed in chicken liver. Therefore, the mode of action of metformin, via AMPK activation, may be different in the chicken liver.