OBJECTIVE To investigate the long-term antidiabetogenic effect of glucagon-like peptide 1 (GLP-1) and its influence on diabetic dyslipoproteinemia, patients with NIDDM were treated with GLP-1 subcutaneously for 1 week. RESEARCH DESIGN AND METHODS Twelve patients participated in the study. The 1st week of the study, all of them were on intensive insulin treatment and from day 8, four were randomized to a control group continuing with insulin, and eight to a treatment group where GLP-1 was given at meals together with regular insulin from day 8 to 12. On days 13 and 14, they were only given GLP-1 at meals. NPH insulin at bedtime was given throughout the study. RESULTS In the GLP-1-treated patients, the doses of regular insulin, given to keep a satisfactory blood glucose control, were reduced compared with treatment with insulin only. GLP-1 virtually inhibited the early increase in blood glucose after the meals, whereas an increase of ∼ 2 mmol was seen during an optimized insulin treatment. In agreement with the short half-life of the peptide, 2-h postprandial plasma insulin levels were significantly decreased both at day 12 and 14, suggesting that there was not enough GLP-1 left to stimulate endogenous insulin release and compensate for the decrease in the dose of exogenous insulin. Therefore, the effect of GLP-1 was lost before the next meal, resulting in increased preprandial blood glucose values at lunch and dinner. The concentration of VLDL triglycerides decreased already during the 1st week. This decrease persisted during the 2nd week when GLP-1 was included in the treatment. No changes were observed in the levels of LDL and HDL cholesterol. The LDL particle diameter increased from a mean of 22.3 to 22.6 nm (P < 0.01) in response to insulin treatment. A further increment to 22.9 nm (P < 0.05) was seen after GLP-1 treatment. The LDL particle size did not change in the control group. Lipoprotein lipase activity was decreased by 27% and hepatic lipase was reduced by 13% in the GLP-1-treated group. CONCLUSIONS We confirm the antidiabetogenic effect of GLP-1 in NIDDM patients. This effect was maintained during 7 days, which implies that the patients did not develop tolerance during this treatment period. Intensive insulin treatment, leading to normotriglyceridemia, increased the mean LDL particle diameter, which is likely to lower the risk of future coronary heart disease in patients with NIDDM. Furthermore, an additive effect of GLP-1 is indicated. Hence, this study gives additional evidence that GLP-1 may be useful as an agent for treating NIDDM.