The parallel course of the excretion rates of bromide and sodium ions was demonstrated in adult male and female rats administered simultaneously with potassium 82Br-bromide and 24Na-sodium chloride. The animals were exposed to various intakes of sodium ions accompanied with five different anions: Br-, Cl-, HCO3-, ClO4-, and SCN-. Regardless of the anion accompanying the sodium ion, the excretion rates of 82Br- and 24Na+ ions were proportional to the magnitude of sodium intake in the animals. Hence, we have proved our hypothesis that the biological half-life of bromide depends on the magnitude of sodium intake rather than on the intake of chloride.