Size, incidence, and volume density of atrial specific granules (ASG) in right atrial cells from five animals each of the rat (average weight 210 g), mouse (average weight 28 g), fruit‐eating bat Megaloglossus woermanni (BMW;average weight 35 g), and the insect‐eating bat Pipistrellus pipistrellus (BPP; average weight 6 g) have been compared via ultrastructural morphometry. In all three parameters of granule measurement, significantly higher figures were obtained in the rodents than in the bats. However, between the rat and the mouse, as also between the two species of bats, no significant differences were noted in any of the measurements. These results therefore do not support the prevalent view that the number and size of the granules decrease with increase in size of the animal species. The low content of ASG in atrial cells of the bats is probably an indication of low demand for the natriuretic hormone of the granule, because, in such animals, and particularly in flight, conservation of fluid and electrolytes is of paramount importance. This suggests that granule content is adapted to fluid and electrolyte regulation in relation to the functional capacity of the animal. We also observed ASG‐like sgtructures in endothelial cells of capillaries of bat tissue but not in rodents. The function of these granules or whether or not they represent atrial specific ones is not clear from the present study. © Wiley‐Liss, Inc.