Fast Ca 2+ -dependent inactivation (FCDI) is a safety mechanism limiting Ca 2+ entry through some types of Ca 2+ channels, including Ca 2+ release-activated Ca 2+ (CRAC) channels. This type of inactivation is caused by Ca 2+ , which passes through Ca 2+ channel and binds to a specific site within a short distance from the inner mouth of the pore, causing channel to shut. The main technique that is used to investigate FCDI is whole-cell patch clamping. Since the cloning of the molecular components of the CRAC channel, STIM1 and Orai1, FCDI of CRAC channel has been studied using HEK293T heterologous expression system. In this paper we describe a method of quantifying CRAC channel FCDI by using instantaneous tail currents.