We propose to synthesize and test new fluorescent indicators for the lithium ion in aqueous solution. The indicators are designed to be water soluble and to respond selectively to Li+ by undergoing a shift in absorption and emission wavelengths. The proposed compounds will be of primary interest for determining the concentration of cytosolic Li+ and for monitoring the flux of Li+ in single cells acted on by neurotransmitters. The indicators will be of use to cell physiologists and neuroscientists studying the influence of Li+ on the phosphatidylinositol cycle implicated in the mechanism by which Li+ functions in the treatment of manic-depressive psychosis. The indicator may also be of use for measuring the concentration of Li+ in the serum of patients undergoing therapy.