The effect of increased dietary sulphur, copper and molybdate on the metabolism of 99Mo administered via the rumen or duodenum was examined. Duodenally administered 99MoO4= was efficiently and rapidly absorbed and excreted via the urine; dietary modifications did not alter this pattern although increased dietary S increased the excretion rate. The passge of 99MoO4= through the rumen led to chemical modification, so that, particularly at high S levels, most of the radioactivity remained in the feces; increased dietary Mo led to increased absorption whereas increased copper decreased absorption. Incorportion of radioactivity into the TCA insoluble plasma fraction was shown with increased dietary S and particularly when both dietary S and Mo were increased. The compatiblity of the results with the hypothesis of ruminal thiomolybdate formation and subsequent reaction with dietary or tissue copper is discussed.