Karst Dace (Telestes karsticus Marčić & Mrakovčić, 2011) is a stenoendemic species of the Danube basin that lives in the Sušik Sinkhole and in the Sušik Creek. Telestes karsticus forms a permanent cave population and the presence of a specific surface population makes it a potentially important model system for understanding the initial adaptive processes that occur during the colonization of caves. In addition, cavefish have evolved differences in behavior which are under the control of the catecholamine systems. The aims of this thesis was to adapt and validate the current method for measurement of monoamine concentrations by HPLC, to quantify catecholamine concentrations in samples of brain and body tissue of the Karst Dace, to determine whether there is a difference in catecholamine concentrations between cave and surface populations, to determine whether there is a change in catecholamine concentrations in individuals exposed to completely dark conditions vs. light/dark conditions and whether these conditions lead to different responses in cave and surface populations. To achieve this goal, it was necessary to purify samples by solid phase extraction. Therefore, the RP-HPLC method for the analysis of compounds containing catechol moiety was developed and validated. The cave population of the Karst Dace shows a trend of higher concentration of dopamine in the brain compared to the surface population, indicating that changes in neurotransmitters could play an important role already in the initial adaptive processes that occur during cave colonization.