This report is divided in two parts. The first section shows that vesicles derived from the sarcoplasmic reticulum of rats skeletal muscle can cleave ATP at a faster rate and produce more heat that the vesicles derived from rabbit skeletal muscle. In the second part, we compared the rates of Ca2+ transport and ATP hydrolysis by rats and rabbits heart sarcoplasmic reticulum. It is shown that the two vesicles preparations are able to use glucose 6-phosphate and hexokinase as an ATP regenerative system. The rates of Ca2+-uptake and ATP hydrolysis measured with glucose 6-phosphate and hexokinase is four to six times slower than that measured with phosphoenolpyruvate and pyruvate kinase as ATP regenerative system. (c) 2008 Elsevier Inc. All rights reserved.