When lambs were fed a cobalt-deficient whole barley diet there was a rapid and massive increase in rumen succinate concentrations. Within 2 d of feeding the Co-deficient diet, the rumen succinate concentrations rose 200-fold and peaked at a level 1000-fold higher than that in Co-sufficient controls. Rumen propionate concentrations decreased, suggesting that an alteration in the balance between succinate- and propionate-producing microorganisms had occurred. The rumen succinate can be absorbed and thus may lead to elevated plasma succinate concentrations in Co-deficient animals, whether fed barley or grass. Thus, the absorbed succinate can at least partially overcome the effect on gluconeogenesis of a decreased activity of methylmalonyl CoA mutase induced by Co-deficiency. These findings suggest that impaired propionate metabolism may not be the primary metabolic defect in ovine Co-deficiency.