A classification of the sarcoplasmic reticulum (SR) membrane into areas devoted to different functions is described. The junctional (j) SR is covered by feet, it faces towards the transverse tubules, and it is probably devoted to receiving a signal from them. The free (f) SR contains the calcium pump. Isolated SR forms two fractions: light and heavy SR. The heavy fraction is composed of the lateral sacs of the triad, containing calsequestrin, and its membrane comprises both f and j SR. The light fraction is entirely composed of fSR membrane. It is proposed that the feet join jSR membrane to particles contained within the transverse tubules and that they play a direct role in excitation-contraction coupling.