Glomerular mesangial cells are known to express angiotensin II type 1 receptors and contract in response to circulating and/or locally produced angiotensin II. In addition, stimulation of mesangial cell matrix protein synthesis by elevated levels of angiotensin II is known to contribute to the development of glomerulosclerosis. Previously, we reported that mesangial cells were positively immunostained with antiserum directed against aminopeptidase A, the principal angiotensinase in the metabolism of angiotensin II. Here we demonstrate directly that aminopeptidase A is expressed in mesangial cells cultured from rat kidney. First, cultured mesangial cells had measurable aminopeptidase A enzymatic activity. Second, immunoblots for aminopeptidase A were positive for isolated glomeruli and mesangial cells, although two bands were seen for mesangial cells (approximately 138 and 144 kD), and only the larger band was seen for isolated glomeruli and kidney. Third, Northern blot hybridizations of total RNA from mesangial cells or kidney were positive and labeled similarly sized bands. Fourth, reverse transcription-polymerase chain reaction amplification of mesangial cell total RNA yielded a partial cDNA of the expected size that was confirmed by sequencing to be identical to rat kidney aminopeptidase A. These results indicate that aminopeptidase A is expressed within mesangial cells. These results further suggest that metabolism of angiotensin II by aminopeptidase A could play a protective role in minimizing the adverse effects of angiotensin II stimulation of mesangial cells.