Skeletal muscle of guinea pigs is composed of at least three types of fibers which are distinguishable by their kinetic and histochemical properties. Fast twitch muscles may be histochemically red or white; fragmented sarcoplasmic reticulum (FSR) prepared from both types of fast twitch muscles has identical calcium-accumulating capacity (maximal calcium uptake) and calcium-concentrating ability. FSR from slow twitch muscles accumulates less total calcium, but the calcium-concentrating ability is equal to that of fast twitch muscles. The slower rate of calcium accumulation by FSR of slow twitch muscles correlates with a prolonged relaxation time.