The mechanism of water conservation is impaired in ageing mammals. An age-related defect in the release of vasopressin has been implicated but, more recently, attention has moved to the renal component of the water conservation mechanism. Previous studies using renal cells prepared from mice of different ages have shown that the threshold dose of vasopressin required to elicit a significant rise in cyclic AMP (cAMP) was greater in older animals. The dose–response curve was moved to the right in 35-month-old mice, i.e. the concentration of vasopressin required to give maximum cAMP output was increased. To investigate this further we examined the binding of vasopressin to renal medullary cells maintained in short-term culture, to determine whether the decreased response of cAMP levels to vasopressin is due to changes in hormone-receptor interaction. In 6-month-old male mice the dissociation constant (Kd) was 2·38 nmol/l and the maximum binding of the hormone (Bmax) was 47·6 fmol/106 cells, and at 30 months of age Kd was 2·37 nmol/l and Bmax was 47·0 fmol/106 cells. In female mice the changes were more complicated because the data for the 6-month-old mice could be split into two groups. It is concluded that there are no age-related differences in the numbers of receptors or their affinity for vasopressin, and that the decreased cAMP response is probably associated with post-receptor mechanisms in this species.