Many animals including cattle can synthesize vitamin C from glucose. The objective of this study was to investigate plasma vitamin C concentration in ketotic cows during the early lactation period because glucose supply for vitamin C synthesis might be limited in these cows. We measured plasma beta-hydroxybutyrate (BHBA) concentration in 118 cows within 2 months after parturition. Subclinical/clinical ketosis was quantitatively determined using a plasma BHBA threshold of 1,200 microM. Plasma glucose concentration was lower in the ketotic cows than in the control cows but plasma vitamin C concentration did not differ between the control and the ketotic cows. Then we measured plasma vitamin C, BHBA and glucose levels in 7 cows during the periparturient period. Plasma BHBA increased and plasma glucose decreased after parturition but plasma vitamin C did not change. These results indicate that plasma vitamin C is not related to the incidence of ketosis in the early lactation period. We suggest that ketotic cows have the ability to produce vitamin C to meet its requirement in the early lactation period although glucose supply is not sufficient for milk production. Vitamin C synthesis is possibly given a high metabolic-priority for glucose in lactating cows.