Vessel diameter, flow velocity, temperature, and viscosity are among the factors determining blood flow in vessels. To assess the influence of arterial blood flow on veins in a cardiac operation, the viscosity of blood diluted with priming solutions must be known. This study aimed to demonstrate the alteration of blood viscosity following dilution with priming solutions: hydroxyethyl starch (HES), gelatin, albumin, and Ringer's lactated solution (RL). Sixty volunteers were (15 female, 45 male) included in the study. Using a 50-ml syringe containing heparin, 40-ml fasting blood was drawn from antecubital veins. Six milliliters of blood was added to glass tubes previously filled with 2 ml of HES 6%, gelatin 4%, albumin, and RL solutions. Relative viscosity values of these mixtures were measured with an Ostwald viscometer device while the temperature was kept constant at 15 degrees C and 30 degrees C by means of a water bath. Measured viscosity values were compared with the calculated viscosity values and corrected values were obtained. All mixtures demonstrated decreased viscosity; however, it was evident that the mixtures containing HES and gelatin presented a lesser degree of decrease in viscosity when compared with the mixtures containing albumin and RL solution. Viscosity of all mixtures except RL decreased significantly at deep hypothermia (15 degrees C).