The purpose of this study was to determine which urine electrolytes should be measured to confirm that the extracellular fluid (ECF) volume is depleted. ECF volume contraction was induced by furosemide administration to rats consuming an electrolyte-free diet. An external potassium balance was achieved by replacing potassium losses with KHCO3 and KC1 so that the sodium and chloride deficits were comparable (equivalent to a 30% reduction in ECF volume). As expected, the urine sodium and chloride concentrations fell to 2 ± 0.3 mmol/l and 3 ± 0.3 mmol/l, respectively. Rats were then randomized to receive 50–75% of their sodium or chloride deficit as either: NaCl (control group), NH4Cl or NaHCO3 to mimic clinical situations associated with ECF volume contraction. In the NaCl group, the urine sodium and chloride concentrations remained low (6 ± 2 mmol/l and 7 ± 2 mmol/l), consistent with persistent ECF volume contraction. Although the NH4C1 group continued to have a low urine sodium concentration (2 ± 0.2 mmol/l), there was now a marked increase in the urine chloride concentration (51 ± 7 mmol/l; p < 0.01 vs. NaCl group). In contrast, although the NaHC03 group continued to have a low urine chloride concentration (2 ± 1 mmol/l), there was a significant increase in the urine sodium concentration (19 ± 3 mmol/l; p < 0.01 vs. NaCl group). We conclude that the clinical assessment of ECF volume by urine electrolytes requires an evaluation of both the urine sodium and chloride conce