The serum osmolarity mainly results from the inorganic ions and the small molecules and only in small extent from the protein ions. In serum sodium, potassium, chloride, bicarbonate, urea and glucose are the only components present in high enough concentrations to individually affect the osmolality. There are several different formulas for the calculation of human serum osmolarity. It has not been demonstrated which of the formulas is most effective for serum from various animal species. The goal of this study is to estimate the serum osmolarity in pigs from the concentrations of the main electrolytes and the glucose and urea content, and to establish the contribution of each osmotic component. Linear regression analysis was carried out to determine the best predictors of serum osmolarity in chickens. Two equations were also deduced for calculating serum osmolarity using manual regression analysis: y = 1.8117 Na + Urea + Glucose + 26.05 and y = 1.8933 (Na+K) + Urea + Glucose + 4.93.