In accordance with their vital role in cardiovascular physiology () corpuscles of Stannius (CS) from two teleosts and an holostean species showed marked and consistent degranulation and exocytotic responses to hypotensive stimuli. In eels (Anguilla rostrata LeSueur) acute blood withdrawal (hypovolemic hypotension) was followed by a prompt decrease in cardiac output (CO) and dorsal aortic pressure (P(DA)), a compensatory tachycardic response and an increase in systemic vascular resistance (R(SYS)). Isovolemic hypotension induced by papaverine i.v., led to a similar, but more prolonged, decrease in P(DA) but the heart rate (HR) continued to accelerate, thereby counterbalancing the severe and persistent decrease in R(SYS). Both hypovolemic and isovolemic hypotension were followed by a significant depletion of cytoplasmic granules from eel CS even though plasma concentrations of Ca, Mg, Na(+), and K(+) were normal. In an ancient holostean fish, the bowfin, Amia calva and a generalized teleost fish, Catastomus commersoni, the number of cytoplasmic granules decreased by 39% and 54%, respectively, 120 min after the acute withdrawal of 8 ml kg bw(-1) of blood. These findings suggest that a primary role of the CS is to release cytoplasmic granules containing renin or isorenin into the blood circulation, in response to hypotension and/or hypovolemia.