The influence of the intestinal microbial reduction of rhein anthraquinone on the formation of deterioration products was studied. Therefore [14C]rhein and [14C]rhein anthrone were mixed with sterilized or non-sterilized cecal mass of rats and incubated for 20 hours at 37 degrees C. Extractions with a methanol-water (50:50) mixture or 4-nitroso-N,N-dimethylaniline (0.1%) in pyridine revealed several radioactive derivatives after TLC and autoradiography, except in the case where the anthraquinone was mixed with sterilized cecal content. Gel permeation on a styrene-divinylbenzene copolymer column of an methanol/water extract of non-sterilized cecal content incubated with [14C]rhein, showed radioactive deterioration products with a molecular weight higher than rhein anthraquinone. The high molecular weight of some deterioration products was confirmed by an ultrafiltration study where the methanol/water extract was centrifuged on a Centricon-3 microconcentrator (nominal cutoff: 3000 MW). Aqueous extracts of non-sterilized cecal content incubated with rhein were extracted with chloroform to remove rhein anthraquinone, rhein anthrone and sennidins before being intracecally injected in rats. No laxative activity was found. Furthermore it was shown that the deterioration products which are probably formed through radical reactions, no longer develop a color with a solution of KOH. Therefore it is concluded that the reduction process of dihydroxy-anthraquinones in the gut microflora followed by an extraction, accounts for the loss of anthranoid equivalents in in vivo circumstances, as several times reported in the past.