Abstract When [1-14C]dodecyl sulfonate and [1-14C]hexadecyl sulfonate were administered orally and ip to free-ranging rats and iv to anesthetized rats, the major route of excretion of radioactivity was urinary. Small amounts only of parent surfactant appeared in the feces of animals dosed orally with [1-14C]dodecyl sulfonate, but large amounts of [1-14C]hexadecyl sulfonate were found in the feces of animals dosed similarly with this surfactant. Thus, fecal radioactivity represents incomplete absorption of material from the gastrointestinal tract, the permeability of which may depend upon the water solubility of the administered surfactant. The pattern of excretion with either compound was essentially unchanged when animals were pretreated with an antibiotic, and thus the gut flora play no significant part in the metabolism or absorption of these surfactants. No biliary elimination with either surfactant was detected following the iv administration, confirming that the fecal radioactivity represented unabsorbed material. The intraperitoneal administration of dodecyl [35S]sulfonate to rats and analysis of the urinary products showed that no desulfonation takes place. Analysis of the urine of rats following the administration of either [1-14C]dodecyl sulfonate or [1-14C]hexadecyl sulfonate revealed the same single metabolite from both surfactants. This metabolite was purifed and identified as butyric acid 4-sulfonate by radioactive glc, mass spectrometry, and cochromatography of its dimethyl derivative. The identity of the urinary metabolite indicates that both dodecyl sulfonate and hexadecyl sulfonate are degraded in the rat by ω,β-oxidation. Whole body autoradiography demonstrates that the liver is the major site of metabolism regardless of the route of administration.