The activities of monoamine oxidase-A and -B were determined in four brain regions (limbic system, occipito-temporal cortex, hemispheres and striatum) of the rat 0, 3, 6, 9 and 14 days after hemitransection of the left side. No larger or consistent change in the activity of monoamine oxidase-A towards 5-hydroxytryptamine was found for the left (hemitransected) side with respect to the right side for any of the rats. The monoamine oxidase-B activity towards beta-phenethylamine increased in the left side striatum to a significant level by 3 days, and in the hemispheres and occipito-temporal cortex on the left side, with respect to the right side by 9 days, but no significant changes were found for the limbic system. A small decrease in the activity of succinate dehydrogenase was found in the striatum on the left side by 9 days after hemitransection, but no change in the activity of acid phosphatase was found in this brain region.