Acid Regular insulin (ARI) and neutral Regular insulin (NRI) were compared in normalfasting subjects by means of a Latin-square experimental design. Both blood glucose and serum immunoreactive insulin were measured after the administration of 0.1 and 0.2 U. per kilogram intravenously, and 0.1, 0.2, and 0.3 U. per kilogram subcutaneously. No statistically significant differences were detected between ARI and NRI. Analysis of blood glucose and serum insulin data pooled from the ARI and NRI treatments disclosed resistance to increasing doses of insulin as the blood glucose fell below 50 to 60 mg. per 100 ml. Thus, relatively small increments in hypoglycemic activity occurred when the intravenous dose was doubled from 0.1 to 0.2 U. per kilogram or the subcutaneous dose increased from 0.2 to 0.3 U. per kilogram in spite of doubling the hypoglycemic effect when the subcutaneous dose was doubled from 0.1 to 0.2 U. per kilogram. In addition, for each 0.1 U. per kilogram of subcutaneouslyadministered insulin the maximum serum levels were approximately 15 to 30 μU. per milliliter andthe duration of effect was extended by one hour. Therefore, following 0.1 U. per kilogram the maximum effect was over by three hours, after 0.2 U. per kilogram by four hours, and 0.3 U. per kilogram by five hours. These results demonstrate the feasibility of studying the time action of insulin in normal subjects and document the conclusion that duration of effect is a function of the dose given.