An improved automated method for determining vitamin C in plasma and tissues is described. Total vitamin C was determined after oxidation to dehydroascorbic acid by reaction with 2,4-dinitrophenylhydrazine. The recovery of ascorbic acid added to plasma was 98 to 101% with a coefficient of variation of 1.8%. Interaction between samples with low, medium, and high concentrations was 1% and the coefficient of variation on 60 replicate analyses of plasma was 2.0%. In the analysis of 16 samples of rat plasma, results obtained by the automated method were essentially the same as those obtained by the original manual method. It was possible to determine vitamin C in 0.15 ml of plasma containing as little as 1.2 μg/ml. Vitamin C was also measured in extracts of rat heart, spleen, kidneys, adrenals, liver, and brain. © 1979.