Specific age-related changes in blood variables of calves have previously been reported. The very first hours after birth are however not fully investigated, and results originating from different breeds are combined. The purpose of this study was to investigate the variation in biochemical variables during the first 48 hours after birth in Holstein-Friesian (HF) and Belgian Blue (BB) calves. Nineteen HF calves born vaginally and 23 BB calves delivered by caesarean section were sampled within 30 min after birth, and at 24 and 48 h of life. The concentration of albumin, chloride, sodium, potassium, calcium, phosphate, urea, creatinine, glucose, beta-hydroxybutyrate, total protein, and activity of AST, gamma GT and glutamate dehydrogenase were evaluated. In both groups, significant decreases were recorded at 24 and/or 48 hours compared with 30 min for albumin, calcium, chloride and creatinine, while significant increases were found for AST, gamma GT, bilirubin, GLDH, glucose and total protein. Changes in analyte concentrations or activities, followed the same trend in both groups, thus suggesting typical features of the newborn calf maturation. The first 24 hours after birth seem to represent a temporal key point in the newborn calf's life for switching from maternal dependence to a self-sufficient and independent survival. This study confirms that age-specific values should be considered for precise interpretation of laboratory results of newborn calves.