1. The metabolism of 3-amino[36Cl]chloropropan-2-ol (III) was studied in male rats. Three urinary metabolites were isolated and identified as alpha-chlorohydrin (I), beta-chlorolactic acid (V) and oxalic acid (VI). Much of the administered aminochloropropanol was excreted unchanged in the urine; 63% within 72 h, 75% over 250 h. 2. Monoamine oxidase is capable of converting aminochloropropanol to beta-chlorolactaldehyde (IV) which, by processes of either reduction or oxidation, suggests that the metabolic pathway is IV leads to I and IV leads to V leads to VI. 3. As assessed by the diuretic activities of the isomers of aminochloropropanol, oxalate appears to be produced by the (+)-isomer but not by the (-)-isomer. A difference in metabolic rate or route of the isomers may account for their differing physiological activities. 4. (+)- and (-)-aminochloropropanol exhibited identical in vitro inhibitory activities on the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and triosephosphate isomerase, and were substrates for monoamine oxidase to equivalent extents.