Whole body radiometry was employed to monitor 22Na radionuclide metabolism in Wistar rats of either sex, aged six months and 1.3 years, kept under normal conditions or on a low-protein diet or during chronic salt load over a period of one year. In all the groups studied, the differences in sodium metabolism according to sex and age were determined. Tissue sodium metabolism was statistically significantly slowed (p less than 0.001) in male and female rats on a high-salt diet compared with the control group. Renal sodium excretion was significantly increased in the high-salt diet group. In the low-protein diet group, tissue sodium metabolism was significantly faster than in control animals, and this both in female (p less than 0.01) and male rats (p less than 0.001). Renal sodium excretion was, however, significantly lower in male rats (p less than 0.001) than in control animals.