AbstractThe occurrence of granular epithelioid cells in the kidney arterial vessels was studied in one‐and two‐year‐old Atlantic salmon during the physiological fresh‐ and seawater periods. The purpose of this study was to make long‐term comparison on the morphology of the renin angiotensin system in the same fish species. One‐year‐old salmon living in freshwater had a statistically significant higher number of granular epithelioid cells (39.9 ± 8.3/mm arterial vessel) than the two‐year‐old fish living in seawater (29.8 ± 5.2/mm arterial vessel,P<0.00001). There was also a significant difference from month to month between the groups (P0.07 freshwate,P<0.3 seawater). With the electron microscope the granules were found evenly distributed within the cytoplasm. They were of high electron density and lined by a single membrane. The granules were composed of a finely granular material. The recorded data on length and weight showed that all fish ate and developed normally. From our results and the available literature, we conclude that in primitive vertebrates, the renin angiotensin system is primarily involved in renal circulation, with vasoconstriction on the afferent