The invention refers to the field of medicine and can be applied as a substance capable of regulating glucose level while treating and preventing diabetes mellitus. There is proposed a biologically active new tetrapeptide lysyl-glutamyl-aspartyl-tryp-tophane of general formula Lys-Glu-Asp-Trp-NH.sub.2 capable of regulating the glucose level, and pharmacological substance containing an effective amount of tetrapeptide lysyl-glutamyl-aspartyl-tryptophane of the general formula Lys-Glu-Asp-Trp-NH.sub.2. There is proposed the method of prevention and/or treatment of the diabetes mellitus, which consists in administering to a patient of the pharmacological substance, containing as an active peptide agent an effective amount of Lys-Glu-Asp-Trp-NH.sub.2 tetrapeptide in doses of 0.1-30 .mu.g/kg of the body weight at least once a day for a period necessary for attaining a therapeutic effect.