The distribution of chrome-alum-haematoxyphil material in the hypothalamoneurohypophysial system has been studied in young (3-6 months), old (12-24 months) and senile (over 24 months) rats. No difference could be detected between the abundance of 'neurosecretory' material in 'young' and 'old' animals, but there was a decrease in the senile rats. The neural lobes of young and old rats contained similar amounts of antidiuretic hormone. It seems likely from this and other evidence that changes in the water metabolism of old rats are due to regression of renal functions rather than to alterations in the rate of synthesis, storage and release of the neurohypophysial hormones.