The paper analyzes the results of hydrological measurements carried out in Modro jezero (Blue Lake) at Imotski in the period from 6 November 2009 at 10:26 p.m. to 4 September 2010 at 3:26 a.m. (a total of 7230 hourly data, or 302 full days). These were continuous measurements of the lake's water depth, water temperature and electrical conductivity. The measurements were carried out using the instrument CTD DIVER (Schlumberger Water Services). The instrument was placed on the lake's bottom, thus the measurements were continuously carried out in one point. The analyzed measurements represent the first systematic, continuous monitoring in this fascinating karst phenomenon. As part of hydrological processing, data on daily precipitation and mean daily air temperatures measured at the meteorological station Imotski were used. Mean hourly and mean daily flows into and out of the lake were defined. It was determined that the lake mostly recharges from a karst aquifer whose dimensions and characteristics have not been sufficiently studied.; U članku su analizirani rezultati hidroloških mjerenja izvršenih u Modrom jezeru kod Imotskog u razdoblju od 6. studenog 2009. u 22 h i 26 min do 4. rujna 2010. u 3 sata i 26 min (ukupno 7.230 satnih podataka) ili 302 kompletnih dana. Radi se o kontinuiranim mjerenjima dubine vode jezera, temperature vode i električne vodljivosti. Mjerenja su izvršena instrumentom CTD DIVER (Schlumberger Water Services). Instrument je bio postavljen na dno jezera tako da su mjerenja vršena stalno u jednoj točci. Analizirana mjerenja predstavljaju prvi sustavni, kontinuirani monitoring na ovom fascinantnom krškom fenomenu. U okviru hidroloških obrada korišteni su podaci o dnevnim oborinama i srednjim dnevnim temperaturama zraka mjereni na meteorološkoj stanici Imotski. Definirani su srednji satni i srednji dnevni dotoci i istjecanja u i iz Modrog jezera. Utvrđeno je da se vode jezera glavninom prihranjuju iz krškog vodonosnika čije dimenzije i svojstava nisu dovoljno izučeni..