1. The 298 patients subjected to angiography were randomized by the sealed envelope system into 3 groups. 2. Group A received no preliminary hydration. Group B received 500 cc. 5 per cent glucose in distilled water immediately prior to angiography. Group C received 500 cc. 10 per cent low molecular weight dextran in 5 per cent glucose immediately prior to angiography. 3. Serial measurements of venous blood hematocrit and viscosity showed changes consistently associated with abnormal red cell aggregation in Group A and Group B only. 4. No statistically significant difference in the incidence of complications was noted between Group A and Group B, indicating that hydration alone has no prophylactic value. 5. Statistically significant reduction in the incidence of complications was noted in Group C, when compared with either Group A, Group B, or Group A and Group B combined. 6. Low molecular weight dextran given intravenously as a 10 per cent solution in 5 per cent glucose immediately prior to angiography significantly reduced the incidence of complications from these procedures. 7. The infusion program described produced no complications or reactions. 8. Pre-angiographic infusion of low molecular weight dextran can be recommended as a safe and effective means of reducing the hazards of arteriography.