Salinity and oxidative stress can transiently elevate cytosolic free Ca(2+) ([Ca(2+)](cyt)) of plant cells. The [Ca(2+)](cyt) increase may be part of a signaling cascade or cell death, depending on cell type, the magnitude and the duration of stress exposure. Several approaches for determining [Ca(2+)](cyt) responses are available to plant biologists, but some require highly specialized equipment. Here we describe protocols for using aequorin as a standard [Ca(2+)](cyt) reporter, with output detected with a plate-reader luminometer.