The effect of diets containing excess calcium and sodium (5 and 10 times above ARC-recommended levels) on the concentration of Ca, Mg, Zn and Cu in rumen fluid of sheep was studied. The concentrations of Ca, Mg and Zn were closely correlated with the decline in pH after feeding. Ca and Mg concentrations remained elevated for up to 6 h after feeding, as compared with preprandial levels. The concentrations of Zn and Cu increased immediately after feeding, but 2-3 h later rapidly decreased to pre-feeding levels. This points to high microbial uptake and/or particle association of microelements in the rumen. Hence, mineral concentration in rumen fluid not always reflects differences in mineral intake. Sodium loading increased Mg concentration, while Ca loading elevated the Cu concentration in rumen fluid. Both tended to decrease the Zn concentration, also the Zn intake in these groups was higher.