The activity of three catecholamine-metabolizing enzymes, monoamine oxidase type A and type B (MAO-A and MAO-B) as well as catechol-O-methyltransferase (COMT), were estimated in homogenates of human spinal cord using radiometric assays. The enzyme activities were determined in postmortem spinal cord tissue from controls and cases with amyotrophic lateral sclerosis (ALS). The activity of MAO-A was below the limit of detectability in both controls and ALS cases. The activities of MAO-B and COMT were evenly distributed at the various spinal levels. The MAO-B activity was substantially elevated in ALS spinal homogenates, whereas only a slight, but not statistically significant, increase in COMT activity was observed. A significant correlation between COMT and MAO-B activities was observed for controls. However, this covariation was not apparent for the ALS cases. These results suggest that the two enzyme proteins are regulated by more complex mechanisms in the spinal cord in amyotrophic lateral sclerosis than simple general increases caused by elevated astroglial cell numbers. In addition, the MAO-A, MAO-B, and COMT activities were estimated in spinal cords from rats treated with the selective MAO-B inhibitor L-deprenyl, a drug with putative neuroprotective effects in neurodegenerative disorders. After 3 weeks of L-deprenyl treatment (0.25 mg/kg/day, sc), the spinal MAO-A and MAO-B activities were decreased by 50 and 80%, respectively. In contrast, the COMT activity was not altered by L-deprenyl administration.