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The diminished sympathomimetic pressor activity of monohydroxylated phenylalkylamines after oral administration has been attributed to incomplete enteric absorption. Therefore, urinary excretion of the unchanged drug and its metabolites has been compared after intravenous and oral administration of 3H-m-octopamine to eight patients. Identical amounts of 3H-activity (80% of the dose) were excreted after the two routes of dosing, so enteric absorption has been assumed to be complete. Significant differences were found in the fraction of free urinary m-octopamine, which amounted to 10.5% of the dose after infusion and 0.58% after oral administration. The only metabolic pathways for m-octopamine are deamination and conjugation. Following oral administration the percentage of conjugates was considerably higher than after intravenous infusion. This metabolic pattern appears typical of all phenylalkylamines with a hydroxyl group in the meta position. Ring hydroxylation to catecholamines was not observed. The enzymes mainly responsible for conjugation after oral administration are located in the gut wall. The resulting ""first pass effect'', i.e. metabolism prior to the access to the central compartment, can account for the diminished pharmacodynamic effect after dosing by this route.

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