The Ca2+-Mg2+-dependent adenosine triphosphatase activity of isolated skeletal muscle sarcoplasmic reticulum was studied in the presence of the lanthanide ion, Gd3+. This ion is a powerful inhibitor, producing half maximal effect at approximately 100 μM Gd3+. Electron microscopy of the isolated vesicles incubated with 100 μM Gd3+ reveal that electron dense deposits of Gd3+ are localized at the site corresponding to the vesicle membrane and that little or no Gd3+ is taken up within the vesicle's interior. This visualization of Gd3+ is apparently dependent on two factors: (i) the presence of ATP, ADP being ineffective; (ii) sufficient time for most of the ATP to be hydrolysed. Since Gd3+ has about the same ionic radius as Ca2+, and since Ca2+ is normally transported across the sarcoplasmic reticulum membrane and accumulated within the vesicle, it is concluded that the increased charge density of the lanthanide ions is critical to the ion transport mechanism, resulting in their localization at the ATPase site and failure to be transported across the membrane.