Since it has been reported that vanadate inhibits Ca⁺⁺-ATPase activity without affecting Ca⁺⁺ uptake, this study was undertaken to investigate the effects of vanadate on Ca⁺⁺-ATPase activity and Ca⁺⁺ uptake in the sarcoplasmic reticulum of rat skeletal muscle. The following results were obtained. 1) Ca⁺⁺ activated ATPase activity of the intact sarcoplasmic reticulum was significantly inhibited when vanadate was added to the incubation medium at concentration greater than 10^-6 M. However Mg⁺⁺-ATPase activity of the intact SR was not affected by vanadate at concentrations ranging from 10^-7 to 10^-4 M. Similarly, Ca⁺⁺-ATPase activity in sonicated sarcoplasmic reticulum was significantly reduced by vanadate at a concentration 10^-7 M or higher. 2) The uptake of Ca⁺⁺ by isolated sarcoplasmic reticulum was also inhibited by vanadate under the conditions where the turnover rate of Ca⁺⁺-ATPase was made to increase. These results suggest that the inhibition of Ca⁺⁺ uptake by vanadate may be correlated with that of Ca⁺⁺-ATPase if experimental conditions are properly set.