The present study was undertaken to measure kidney monoamine oxidase activity (MAO) isolated from three different rat groups: Wistar Kyoto rats (WKY), stroke-free cases of stroke-prone spontaneously hypertensive rats (SHRSP-control) and SHRSP-stroke cases (SHRSP-stroke). The SHRSP group was found to have less kidney MAO than the WKY group. Conversely, plasma norepinephrine concentration of SHRSP group was significantly higher than that of the WKY group (Jpn. Heart J., 25, 833-835, 1985). These findings suggest that the increased blood pressure in SHRSP may be related to increased plasma norepinephrine concentration caused by a decrease in degradation enzyme, MAO. An attempt was also made to determine the effects of clorgyline and deprenyl. No significant difference in MAO was observed between the kidneys isolated from the WKY, SHRSP-control and SHRSP-stroke groups. However, the inhibition curve of clorgyline reached a plateau after producing 33% inhibition of MAO. It was assumed that rat kidney mitochondrial MAO consists of 33% MAO-A type and 67% MAO-B type.