In this study, the efficacy of intraruminal copper boli in the prevention of molybdenosis in cattle caused by environmental contamination was examined. The efficacy and safety of a high- and a low-dose intraruminal copper bolus were tested. Both treatments maintained copper serum levels within normal limits. The high-dose treatment caused very significant (P<0.001) differences in serum copper levels compared with untreated controls. Significant differences could not be demonstrated in the second trial in which the effects of the high- and the low-dose bolus were compared because an unexpectedly high number of animals had to be excluded from the trial. No abnormal high copper levels or signs of toxicity were observed. In order to prevent excessive copper accumulation in the liver, the lower dose copper bolus may be preferable.