There has been a very meager study on the effect of age on the haemobiochemical performance of rainbow trout at different ages. The present investigation focused on the age specific changes in the hematological and biochemical parameters in rainbow trout, Oncorhynchus mykiss. Blood samples collected from 6, 9, 12, 15 and 18 month-old trout (n = 30 for each age) revealed varying results. Values for PCV and RBC indices were significantly lower, and plasma protein concentration was significantly higher in younger fish. Total WBC and lymphocyte counts were significantly higher in fish at 6 and 9 months of age, while neutrophil and monocytes counts were higher at 6, 9, 12 and 15 months. Eosinophil counts were significantly higher in 9 month old fish. Values for most biochemical analytes were significantly different among age groups except for creatinine and potassium concentration.