The excretion and distribution of 14C were studied in rats given by oral intubation 14C-labelled (-)-(1S,3S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid and its stereoisomer, (-)-(1R,3S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (MTCAs), compounds that are known to become mutagenic on reaction with nitrite. For comparison, another group of rats was given by oral intubation products of the in vitro treatment of [14C]MTCAs with nitrite. When rats were given the untreated [14C]MTCAs, most of the 14C was excreted in the faeces within 24 hr and was noted only in the intestine on autoradiograms of rats killed 30 min and 6 hr after dosing. In contrast, when rats were given the nitrite-treated [14C]MTCAs, 14C was mainly excreted in the urine within 24 hr and was noted not only in the intestine but also in the liver and kidney on autoradiogram. These results suggest that the majority of MTCAs pass rapidly through the gastro-intestinal tract, while the bulk of the reaction products of MTCAs with nitrite are absorbed from the gastro-intestinal tract and excreted in the urine.