An automated high-performance liquid chromatographic method for the determination of the diuretic drug furosemide has been established. Dog plasma was injected directly into a two-column system with a BSA-ODS (ODS column coated with bovine serum albumin) precolumn and a C18 analytical column for the separation of furosemide. The two columns were automatically switched. Furosemide remained trapped on the precolumn while proteins were eluted to waste. After column switching, furosemide was washed onto the analytical column and analysed without interference. The greatest advantage of the method is its easy performance without manual sample preparation; it requires no extraction or deproteinization. The method allows determination of 0.1-10 micrograms/ml of furosemide with accuracy and precision comparable with previously reported values. The coefficients of variation obtained from replicate measurements of 1 microgram/ml and 5 micrograms/ml samples were 1.65% and 2.40%, respectively. This method was used to measure the plasma levels of furosemide in beagle dogs to whom the drugs was administered, as a reference, in a toxicological study.