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Thirty-two weanling Hereford heifers were assigned to treatments in a 2(3) factorial arrangement involving two levels of Mo (0 and 100 ppm added inorganic Mo), two levels of SO4 (0 and .5% added inorganic SO4) and two levels of alfalfa pellets (0 and 2.72 kg daily). A basal diet of hay and salt ad libitum and 454 g milo pellets (92% milo and 8% molasses) was fed daily. All animals were fed individually for 11 mo. Added dietary Mo created severe symptoms of Mo toxicity that included scouring, achromotrichia, anemia and weight loss. Five of the 16 animals that received Mo died within 2 wk after the study was terminated. Molybdenum accumulated in all tissues sampled during the first 8 mo and decreased thereafter. Molybdenum also elevated plasma Cu. Balance data indicated that 100 ppm of added dietary Mo led to daily retention of 105 mg Mo and reduced the rate of liver Cu depreciation. Added Mo did not influence feed intake, digestibility or metabolizable energy (ME) when calculated per unit of metabolic size. Added inorganic SO4 lowered plasma Cu and Mo, but did not alter digestibility, ME or Cu and Mo balance. Added protein supplied by alfalfa pellets increased feed intake and digestibility of dry matter, crude protein and energy but did not prevent Mo toxicity symptoms. Alfalfa also increased ME, P, S and N balance and was involved with certain interactions with both Mo and SO4. Plasma Mo is a good indicator of Mo intake and is more useful than tissue Cu levels. Urinary Mo may also be useful to evaluate Mo intake under field conditions.