Kidney dysfunction has been observed in aged humans and rats, the primary cause of which may reside in the kidney itself or in the hypothalamus. The latter possibility is suggested by the increased release of AVP in response to salt infusion in humans. The effect of age on the relationship between plasma osmolality and plasma AVP concentration has never been verified in an animal model. Therefore, in the present study, 9% salt solution was infused into adult and aged Lewis rats, and plasma AVP concentration and osmolality were measured. Basal plasma AVP concentration, osmolality and total water intake were not altered in aged Lewis rats as compared with adult animals, indicating the absence of overt disturbances in water homeostasis. Infusion of 9% salt solution resulted in a linear increase in plasma osmolality in both adult and aged rats. Plasma osmolality increased more with time in aged animals than in adult animals, suggesting an age-related difference in kidney function during salt infusion. Plasma AVP concentration increased 50% less with osmolality at relatively low osmolalities, but not at relatively high osmolalities. The altered relationship between plasma osmolality and plasma AVP concentration in rats with age may be related to changes in neurons monitoring osmolality or to changes in baroreflex regulation. The data suggest that reduced kidney function with age does not result from an altered relationship between plasma osmolality and plasma AVP concentration.