Radiographic contrast materials added to blood reduce the red cell membrane potential by balancing the internal impenetrable anions, hemoglobin and organic phosphates. In so doing, a redistribution of protons occurs such that plasma is acidified. The time course of acidification of plasma is measured in seconds, with a nadir of pH occurring 12 to 15 seconds after addition of Hypaque (1.5 to 3.0 ml/10 ml blood) and a half-time of acidification requiring about 6 seconds. The acidification process is slowed in part by an initial alkalosis due to Hypaque. The. acidification of blood is more rapid after addition of Renografin (1.5 to 3.0 ml/10 ml blood) than after addition of Hypaque since the former solution is slightly acidic. The time course of plasma acidification indicates that a maximal reduction in blood pH may not occur in the capillaries of a regional circulation following injection of contrast materials into its afferent vessel, since the transit time of the contrast material may be less than the time required for maximal acidification of plasma.