Thirteen different brands of 40 mg furosemide tablets available in Thailand were evaluated. In vitro studies revealed that all products met the requirement of United States Pharmacopoeia XX for weight variation, per cent labeled amount, and disintegration time. However, there were only 4 brands that passed the dissolution test specification. The original brand (brand A) and the three local brands (brand B, C and D) with differences in dissolution characteristics were selected for bioavailability study. Relative bioavailability of 40 mg furosemide tablets were performed on eight healthy Thai males using a crossover design. Plasma furosemide concentrations were determined by high-performance liquid chromatographic method. Clinical response to these tablets were also studied by determination of urine output and electrolyte excretion. Individual plasma data was analyzed according to one-compartment open model using the PCNONLIN computer program. The elimination half-life of furosemide was 1.27 hours. The mean individual peak plasma levels ranged from 0.61-1.12 micrograms/ml and the time required to reach the peak ranged from 1.63-2.00 hours. There were no statistically significant differences in parameters studied between the original and the local brands (p greater than 0.05). The relative bioavailability of furosemide with respect to brand A were 70.29, 113.41, and 94.93 per cent for brand B, C and D, respectively. There was no relationship between in vitro and in vivo characteristics. Clinical response, in terms of diuresis and electrolyte excretion, e.g., sodium, chloride and potassium, between four brands of furosemide tablets were also not significantly different (p greater than 0.05), indicating that the four brands are clinically equivalent.