Dopamine, as catecholamine, acts as a neurotransmitter in the central nervous system (CNS) and as a hormone outside the CNS. Disorders of the dopamine system can lead to various diseases such as Parkinson's disease, schizophrenia and essential hypertension. Oxidative stress plays an important role in the pathogenesis of these diseases. Molecules containing a catechol group are good antioxidants, and since the catechol group is an integral part of catecholamines, they play an important role in defending the CNS against free radicals. The reaction of dopamine and DPPH• radical in this study was used as a model for dopamine's reactivity towards free radicals. The aim of this thesis is to examine the kinetics of the reaction of dopamine with DPPH• and to determine the rate constants and the kinetic isotope effect (KIE) to better understand the reaction mechanism. The rate constants of the reaction were determined in solvent mixtures of 1,4-dioxane-H2O (0,95-0,05 v/v and 0,75-0,25 v/v) and 1,4-dioxane-D2O (0,95-0,05 v/v and 0,75-0,25 v/v) in an acidic medium where the cationic form of the dopamine molecule is predominantly present. From the obtained rate constants of the reaction, the KIE was determined under the conditions of 1,4-dioxane-H2O (D2O) (0,75-0,25 v/v). KIE at an HCl (DCl) concentration of 0,00005 mol dm-3 is 8,2, while at an HCl(DCl) concentration of 0,01 mol dm-3 it amounts 15,8. The measured KIE values deviate from the semiclassically predicted values for the OH bond, indicating that the key step in the reaction mechanism under these conditions is hydrogen transfer from the catechol group of dopamine to the N atom of the DPPH• radical. Additionally, these KIE values suggest the presence of the hydrogen tunneling in the reaction between dopamine and the DPPH• radical in a predominantly nonpolar reaction medium.