Monoamine oxidase (MAO) activity (substrate: tyramine) has been studied in rat intestinal wall mitochondrial fractions identified by monitoring succinate dehydrogenase and cytochrome oxidase activities. The MAO activity, which was not due to contamination with mitochondria, has been also found in nuclear and microsomal (+hyaloplasm) fractions. Deamination of tyramine, serotonin and dopamine by rat intestinal mitochondrial MOA obeyed the Michaelis--Mentern kinetics. The Vmax values were the highest for deamination of tyramine, the lowest--for norepinephrine. The lowest Km value was recorded in the systems with 2-phenylethylamine. Data on the inhibitory effect of low concentrations of deprenyl suggest that 50% of the total tyramine deaminating activity in rat intestinal mitochondria was due to presence of MAO type B. Low concentrations of chlorgyline inhibited the deamination of tyramine in these systems by 20-30% suggesting a possibility of presence in the rat intestinal mitochondria of a tyramine deaminating activity distinct from MAO type A. Pyrazidol or harmine, which are selective inhibitors of the MAO type A, caused only partial (30-40%) inhibition of MAO activity (substrate: tyramine) in rat intestinal mitochondria. Controlled heating experiments indicated higher thermostability of MAO type B (substrate: 2-phenylethylamine) as compared with MAO type A (substrate: serotonin) in rat intestinal mitochondria. The data obtained suggest that rat intestinal mitochondria, contrary to human intestinal mucosa (cf. ref. 2), contain about 50% of MAO type B, which is comparatively thermostable and does not resemble in this respect the MAO type B in many other biological sources.