Sheep fed diets containing less than 70 micrograms Co per kg develop vitamin B12 deficiency. When sheep were fed diets containing 20 micrograms Co per kg or less, mean rumen succinate concentrations increased by more than one hundred-fold within 2 days. This increase was matched by an equimolar decrease in mean rumen propionate concentrations. When diets containing more than 20 micrograms Co per kg were fed to sheep, no such changes occurred. The synthesis of succinyl CoA from propionyl CoA in liver is impaired in ovine cobalt deficiency. We suggested that, paradoxically, accumulation of rumen succinate could reduce the effects of vitamin B12 deficiency on methylmalonyl CoA mutase and consequently result in lower plasma methylmalonic acid (MMA) concentrations than would arise by feeding diets that did not affect rumen succinate concentrations. This hypothesis was tested by feeding diets containing 4, 40 and 1000 micrograms Co per kg to sheep for 23 weeks. However, sheep fed 40 micrograms Co per kg did not have mean plasma MMA concentrations that were higher than those in the sheep fed 4 micrograms Co per kg, indicating that rumen succinate accumulation did not ameliorate the effects of Co deficiency.