The concentration of angiotensin converting enzyme (ACE) in the lung, kidney and plasma of the following strains of rats were investigated: Sprague Dawley (SD), Wistar Kyoto (WKY), spontaneously hypertensive (SHR), stroke prone SHR (SP-SHR) and low blood pressure SHR (LBP-SHR) i.e. SHR whose blood pressures were below 150 mm Hg. The enzyme concentrations in the lung of the hypertensive rats were significantly higher than those of the normotensive rats but the reverse was true for plasma ACE concentrations. The kidney ACE concentrations in the hypertensive rats were also lower than those of the normotensive rats, but the differences were not significant. There were no significant differences in enzyme concentrations between the normotensive SD and WKY. Similar results were obtained with the three sub-strains of hypertensive rats despite the fact that their mean blood pressure (MBP) varied significantly from each other.