OBJECTIVE: To study the effects of exercise on the absorption of the basal long-acting insulin analog insulin glargine (Lantus), administered subcutaneously in individuals with type 1 diabetes. RESEARCH DESIGN AND METHODS: A total of 13 patients (12 men, 1 woman) with type 1 diabetes on a basal-bolus insulin regimen were studied. (125)I-labeled insulin glargine at the usual basal insulin dose was injected subcutaneously into the thigh on the evening (2100) before the study day on two occasions 1 week apart. Patients were randomly assigned to 30 min intense exercise (65% peak oxygen uptake [Vo(2peak)]) on one of these visits. The decay of radioactive insulin glargine was compared on the two occasions using a thallium-activated Nal gamma counter. Blood samples were collected at regular intervals on the study days to assess plasma glucose and insulin profiles. RESULTS: No significant difference was found in the (125)I-labeled insulin glargine decay rate on the two occasions (exercise vs. no exercise; repeated-measures ANOVA, P = 0.548). As expected, a significant fall in plasma glucose was observed over the exercise period (area under curve above fasting [DeltaAUC] glucose: -0.39 +/- 0.11 vs. -1.30 +/- 0.16 mmol . l(-1) . h(-1); nonexercise vs. exercise; P = 0.001), but insulin levels did not differ significantly on the two occasions (DeltaAUC insulin: -2.1 +/- 3.9 vs. 1.5 +/- 6.2 pmol . l(-1) . h(-1); nonexercise versus exercise; P = 0.507). CONCLUSIONS: An intense 30-min period of exercise does not increase the absorption rate of the subcutaneously injected basal long-acting insulin analog insulin glargine in patients with type 1 diabetes.