Developing male and female rats were examined for the age of onset of the salt intake behavior and the relationship between the sodium appetite and the plasma ALDO of the rat. Methods: Rats were depleted by a combination of pharmacological natriuresis (using lasix) and maternal deprivation, then rats were infused with 3% salt solution through oral catheters by using controled outmatical pumps; The concentration of plasma ALDO of the rats was analysed by using radioimmunoassay. Result: The salt appetite of the rat was reliably evident from 13 days of age in both male and female rats, but no reliably evident from the age of 12 days and under. There is a significant increase of the plasma ALDO in the salt-depleted rats compare with that of the normal fed rats. Conclusion: The data suggests that all necessary components of the mechanism for the expression of sodium appetite are assembled at about 13 days of age in the rat brain.