Selenite uptake and incorporation in Selenomonas ruminantium was constitutive with an inducible component. It was distinct from sulphate or selenate transport, since sulphate and selenate did not inhibit uptake, nor could sulphate or selenate uptake be demonstrated. Selenite uptake had an apparent Km of 1.28 mM and a Vmax of 148 ng Se min-1 mg-1 protein. Uptake was sensitive to inhibition by 2,4-dinitrophenol (DNP), carbonyl cyanide m-chlorophenyl hydrazone (CCCP), azide, iodoacetic acid (IAA) and N-ethylmaleimide (NEM), but not chlorpromazine (CPZ), N,N'-dicyclohexyl-carbodiimide (DCCD), quinine, arsenate, or fluoride. Treatment of cells accumulating 75[Se]-Selenite with 2,4,DNP inhibited uptake, but did not cause efflux. Transport of selenite was inhibited by sulphite and nitrite, but not by nitrate, phosphate, sulphate of selenate. 75[Se]-Selenite was incorporated into selenocystine, selenoethionine, selenohomocysteine, and selenomethionine and was also reduced to red elemental selenium.