Analysis of nanoliter samples of electrolytes using a flow-through microfluorometer. Several techniques have been developed to study the transport properties of nanoliter samples of renal tubule segments, such as continuous flow colorimetry and continuous fluorometry. We have extended the capability of the NANOFLO™, a flow-through microfluorometer, designed for measurement of carbon dioxide, urea, ammonia, glucose, lactate, etc., to analyze sodium, calcium and chloride ions, using three commercially available fluorescent indicators for intracellular and extracellular measurements. The selection of fluorescent indicator for each electrolyte was dependent on the optimal match of the dissociation constant and the analyte concentration range of interest. Using Fluo-3 dye we achieved a detection limit for Ca²⁺ of 0.1 pmol and selectivity over Mg²⁺ of between 7:1 to 10:1. Using sodium green dye we achieved detection limit for Na⁺ of 12 pmol and a selectivity over K⁺ of 40:1. The detection limit for Cl^- using lucigenin dye was 10 pmol. This technique can be readily adapted for the measurement of other physiologically important ultralow volume.