Five male Gezel lambs received different levels of monensin (0, 10, 30 and 60 mg/kg DMI) with their diet in a cross over design. The experimental diet consisted of 70% concentrate and 30% roughage. Blood samples were taken from the jugular vein 1 hour before and 2 hours after feeding and their sera immediately separated to measure the serumic concentrations of glucose, triacylglycerol, total protein, albumin/globulin ratio, blood urea nitrogen, and also aspartate aminotransfrase (AST) and gamma glutamyltransfrase (GGT) activity. Rumen fluid was also collected by rumenocentesis following feeding and its pH determined. During the experimental period, clinical examination was performed and the results were recorded. Significant differences in mean serumic levels of triacylglycerol, albumin/globulin ratio, blood urea nitrogen and the activity of AST and GGT were not seen before and after feeding although there were significant differences in this respect between different levels of monensin (