Contraction of vertebrate skeletal muscle is caused by calcium ions released from the sarcoplasmic reticulum (see refs 1, 2 for reviews). The ensuing transient change in the intracellular level of ionised calcium has been monitored using various Ca2+ indicators, sich as murexide, aequorin, and arsenazo III. So far, most of what is known about these calcium transient derives from experiments on barnacle or frog muscles fibres, and it is desirable to extend such studies to mammalian muscle. We report here that the photoprotein aequorin can be used to monitor calcium transients in rat and human muscles, and that the transients decay more quickly in fast contracting muscle fibres.