SummaryAtrial granularity was measured using a new morphometric method in rats subjected to procedures known to alter water-electrolyte balance. It was found that water deprivation and sodium deficiency lead to atrial hypergranulation, whereas, the combined administration of desoxycorticosterone (2.5 mg/100 g body wt/day) and 2% NaCl in the drinking water leads to degranulation. A statistically significant correlation was found between atria granularity and hematocrit values in water deprivation; animals with higher hematocrit had more granules. The findings point to a relationship between atrial-specific granules and the regulation of water-electrolyte balance in these situations.