1. NADPH-linked aldehyde reductase from pig, ox and rat brain exhibits non-linear reciprocal plots when partially purified enzyme preparations are studied. 2. In pig brain this non-linearity is due to the presence of two distinct aldehyde reductases, which can be separated by DEAE-cellulose chromatography. 3. These two enzymes can be distinguished by several criteria, including pH optima, Michaelis constants for substrates and their inhibitor sensitivity. 4. The probable role of these enzymes in the metabolism of the aldehydes derived from the biogenic amines is discussed.