The bioequivalence of two oral preparations of the diuretic furosemide, namely (i) a Croatian pharmaceutical product (test preparation A) and (ii) a reference preparation B, both in a dose of 500 mg was assessed in an open, cross-over, randomized trial in 15 healthy male volunteers, in whom the HPLC method with a fluorescent detector was used to determine its concentrations. The test preparation (A) was found to achieve a considerably higher concentration (17.2 +/- 9.304 mg/l) than the reference preparation (11.1 +/- 6.484 mg/l); the time to peak concentrations was statistically significantly shorter for the test preparation (1.033 +/- 0.743 h) than for the reference preparation (1.656 +/- 0.586), and the areas under the concentration curves were statistically significantly greater for the examined preparation (65.9 mg.h/l) than for the reference preparation (46.845 mg.h/l). The relative bioavailability of the test preparation was 129%, i.e. it was not bioequivalent with the reference preparation. This finding was consistent with the previously performed laboratory quality testing in vitro, where the release of the reference preparation was found to be considerably slower and weaker than that of the test preparation. High doses of furosemide exemplified by 500 mg were found to affect only some of the pharmacokinetic parameters, i.e. they induce an accelerated absorption, an increase in serum concentration, and a prolongation of its half-life.