Introduction: Insulin infusions over 2 U/hr can suppress endogenous insulin secretion in healthy subjects 30-45% [1]. Virtually all tight glycaemic control (TGC) protocols deliver insulin via infusion. This study examines the impact of bolus delivery of insulin in TGC on the endogenous insulin secretion of critically ill patients. Methods: 18 patients from the Christchurch Hospital ICU enrolled in a prospective clinical trial studying sepsis each had two sets of blood samples assayed for insulin and C-peptide. The first set was taken at the commencement of the SPRINT TGC protocol for patients with suspected sepsis. The second set was taken when their SIRS score was consistently below 2. Each set had 4 samples taken at: -1, 10, 40 and 60 min following bolus delivery of insulin as required by SPRINT to capture endogenous insulin secretion during the bolus profile. Bolus size was dictated by the protocol, but was in the range 2-6 units. Model-based methods [2] were used to calculate the endogenous insulin secretion rate for each set of samples. The level of suppression was calculated as the ratio of the secretion rate between 5-15 mins (just after peak plasma insulin) and average of the 0-5 min (basal) and 15-60 min (return to basal) secretion rates identified.