Nine 5-month-old lambs were randomly allocated to two groups and were fed on either a Co-deficient whole-barley diet (n 5), or the same diet supplemented with Co (n 4). The lambs were fed on their respective diets for 28 weeks. Plasma vitamin B12 concentrations fell below the lower limit of normality after 6 weeks, and plasma methylmalonic acid (MMA) concentrations rose above the upper limit of normality after 10 weeks. However, plasma MMA concentrations fell to near normal levels towards the end of the experiment suggesting that diagnosis of more severe Co deficiency based on determination of plasma MMA concentrations may be of limited value. Analysis of tissue samples collected at slaughter revealed a marked reduction in the vitamin B12 concentration and the activity of methylmalonyl-CoA mutase (EC 220.127.116.11) in the tissues taken from the Co-deficient sheep, by comparison with the controls. Although tissue concentrations of MMA in the Co-deficient animals were not significantly different from those of the controls, we did detect increased concentrations of branched-chain fatty acids. This suggested that misincorporation of MMA, but not propionic acid, into fatty acids had occurred. The Co-deficient lambs did not develop any neurological signs, suggesting that accumulation of branched-chain fatty acids may not be involved in the development of neurological lesions.