Reproductive performance and lamb viability in cobalt sufficient and subclinically deficient ewes, and from ewes experiencing repletion from and depletion into cobalt deficiency, were investigated in two experiments. The sheep were fed a cobalt deficient ration and supplementation was by oral dose according to treatment. The treatments had a significant effect (P<0·001) on ewe serum vitamin B12 and methylmalonic acid concentrations. There were no significant effects on ewe liveweight, condition score or conception rate. Cobalt deficient ewes produced fewer lambs and had more stillbirths and neonatal mortalities than cobalt sufficient controls. Lambs from deficient ewes were slower to start suckling (P<0·05), had reduced concentrations of serum immunoglobulin G and zinc sulphate turbidity levels (P<0·05), and had lower serum vitamin B12 and higher methylmalonic acid concentrations (P < 0·05), than lambs from cobalt sufficient dams. Cobalt supplementation in either the first or second half of pregnancy only did not fully alleviate these adverse effects.