Statistical Analysis Power Calculation: Data from an earlier trial determined that the mean area under the curve (AUC) for children with T1DM with insulin alone was 470 mg*hr/dl. The current trial is designed to detect a 20% difference in mean AUC for glucose in the 300 min period to 376 mg*hr/dl. In our previous study, r > 0.7 was noted between repeated measures (in same subjects) and that the standard deviation for our excursion measure AUC is approximately 30%. With these specifications the necessary sample size is 11 subjects. With a 40% drop out rate we would need a total of 15 subjects with TIDM. Power calculations were based on an alpha of 0.5 and power of 80% and two-tailed test. To stratify them in to recent onset Vs established onset we will recruit 15 of each (n = 30). Data Analysis: AUC will be determined for glucose for a 300 min time period for all 4 studies and a two-tailed, repeated measures analysis of variance will be performed. A modified paired t-test will be used for post hoc multiple comparison. Regression analysis will be used to calculate a dose response curve for each individual and the hypothesis will hold true at a significance of 0.05. Similar analysis will also be carried out for insulin and glucagon. The pharmacokinetics of Sitagliptin will be analyzed by model dependent and model-independent means. AUC, plasma clearance, half-life and steady state volumes of distributions will be among the parameters determined. Pharmacokinetic data will be analyzed using ADAPT II software. The initial curve fits will be performed with the weighted least squares algorithm. Other strategies will be invoked as appropriate.