Muscle contraction is a phenomena which fascinated already the ancient Greeks. People have long sought to understand the mechanism of muscle contraction. Today we know that in order for muscle to contract, an action potential propagates from the nerve cell to the muscle cell. Upon arriving at the muscle cell, via a mechanism called Excitation- Contraction (E-C) coupling, Ca2 + is released from an intracellular membrane system, the sarcoplasmic reticulum (SR), into the intracellular fluid. The increase of intracellular Ca2 + initiates the interaction between the contractile units which results in force development and tension. The least well understood step in the contractile process is mechanism of E-C coupling. During the last 15-20 years various theories have been proposed to describe this process. Our laboratory came up with a theory several years ago, that critical sultbydryl groups on a protein, the ryanodine receptor(RyR)/Ca2 + release channel, are oxidized and subsequently reduced during the process of contraction and relaxation. In this thesis a reagent, o-Phthalaldehyde (OPA), was used to better understand the gating mechanism of the RyR/Ca2 + release channel. This reagent has the ability to form an isoindole derivative with the amino acids cysteine and lysine, if they are separated by not more than 3 A .In this study, it was shown that OP A interacts directly with the Ca2 + release channel by forming a covalent derivative with a critical thiol and a nearby lysine. High affinity [3H]Ryanodine binding to the RyR\Ca2 + release channel is activated by < 130μM OP A, but is inhibited by OPA at concentrations ranging from 200-300 μM OPA. This biphasic behavior indicates that at least two sets of cysteine-lysine pairs regulate Ca2 + channel activity. Moreover, the binding of OP A results in increasing the affinity of the receptor for the binding of ryanodine, in a Ca2 + independent manner, which may indicate that there are two different sets of RyR\Ca2+ release channels present in the SR.