How effective is sodium fluoride as a preservative of blood glucose? We compared changes in glucose concentration in fluoride-treated blood specimens with those of heparin-treated specimens. The former declined rapidly during the first hour; thereafter the rate of decrease was slower, and after 4 h the glucose concentration in the blood samples remained stable for up to three days. In contrast, the glucose concentration in the heparin-containing samples declined continuously. During the first hour, however, the rates of decline in the two types of samples were similar. Evidently sodium fluoride takes effect slowly but effectively in preserving glucose in blood for at least three days. Its use, however, is unnecessary if the concentration of glucose is to be measured within the first hour after sampling.