Glucagon-like peptide (GLP-1) is widely considered as a potential drug against diabetes mellitus and obesity. It strongly stimulates the pancreas to produce and release insulin, even a few minutes after meal consumption. Because of this action, GLP-1 has been called an "incretin hormone". Moreover, GLP-1 decreases the level of glucose in the blood, independently of insulin. An obstacle to clinical application is the very short half-time of GLP-1 degradation by dipeptidyl-peptidase IV in the blood. This research was aimed at tracing all possible changes evoked by long-term application of GLP-1 in rats and comparison of two methods of application: osmotic minipumps and daily injections. In the 13-day experiment, samples of blood, muscle and liver from 24 male Wistar rats were used. Analysis included glycogen, glucose, triglycerides, free fatty acids, cholesterol, triiodotyronin, thyroxin, insulin and glucagon concentrations. The results show a lack of significant differences between both methods of application. We suggest this may be evoked by adaptation of the organism to the prolonged action of GLP-1.