Vesicular fragments of sarcoplasmic reticulum isolated from rabbit skeletal muscle were actively loaded with Ca2+ in the presence of ATP and an ATP-regenerating system using Arsenazo III as metallochromic indicator to monitor Ca2+ movements across the membrane. Once the Ca2+ release is triggered by the presence of tetraphenylboron in the reaction medium, the addition of verapamil or diltiazem gives rise to a net Ca2+ entry inside the vesicles. Preincubation in the presence of verapamil does not abolish the tetraphenylboron-induced Ca2+ release, the verapamil-induced Ca2+ accumulation being still observed. There appears to be a high-affinity site for verapamil titrated in the micromolar concentration range, whereas diltiazem demonstrates more complex behavior when its concentration is raised. This study suggests the existence of a Ca2+ pathway (putative channels) which is blocked by the drugs tested allowing Ca2+ accumulation inside the vesicles owing to the Ca2+-dependent ATPase activity.