The paper discusses hourly changes in specific conductivity in two lakes and compares them to changes over longer time intervals. The short time intervals between measurements are designed to help assess the course of seawater intrusions. Two lakes on the Polish coast were selected for research purposes – Lakes Gardno and Łebsko. Specific conductivity was measured using an automatic YSI Sontek 6920V2 probe. It was shown that Lake Łebsko has a permanently elevated specific conductivity, whereas Lake Gardno experiences episodes of fluvial influence. The specific conductivity was shown to change constantly in both lakes, as evidenced by multi-day, daily and hourly data.