Results of the preliminary study of influences of the planned highway sector Trebnje - Hrastje on karst waters are summarized. The study was based on field survey and review of previous researches. Underground water flow from the stream Lukovški potok to the Zijalo spring with average velocity 0.45 cm/s and further on to the Prečna spring with velocity 4.1 cm/s was proved. This second part of flow is significantly enlarged by recharge from the karst aquifer of Suha krajina. At high and highest waters rather faster flow can be expected and at the same time also fast spreading of eventual pollution. Velocity and mode of infiltration of rain directly into the karst underground was not defined yet. Based on the results of tracing test the underground connection between the Bršljinski potok and Lukovški potok can be estimated as questionable.