The effects of method of administration of rapeseed oil on total, soluble, and ionizable Ca and Mg in ruminal fluid were determined for four diary cows fed a basal diet without (control) or with rapeseed oil (1 kg/d) added by continuous infusion or by a single administration via the ruminal cannula. The control diet contained 68% forage and 32% concentrate mix (DM basis). Fatty acid, Ca, and Mg contents, respectively, were 1.78, 1.16, and .47% for the control diet and 7.38, 1.05, and .44% for the supplemented diets. Soluble and ionizable Ca and Mg concentrations of ruminal fluid and the total Ca, Mg, and soaps of fatty acids in ruminal contents were determined over 24 h. All concentrations except total Ca varied with sampling time. All contents of soluble and ionizable Ca were decreased by lipid supplementation. A lack of ionizable Ca may limit the attachment of bacteria to particles, thus reducing ruminal digestion. In contrast, variations of Mg from lipid supply were lower than those of Ca. The interaction between time and diet was significant for soluble and ionizable Ca contents of ruminal fluid. Diurnal variation of these components was not associated with variations in the estimated soaps of fatty acids.