In this paper a method of obtaining of the urea biosensor, based on detection of ammonium ions is presented. The preliminary experimental results concerning detection of urea in human plasma are given. Methods of selection of solution for conditioning and storing of urea biosensors are described. Additionally, the results concerning influence of EDTA and Na5P3O10, as additions for tris/HCL buffer, on permanence of the urea biosensor are presented.