Optical configuration and stirring arrangement for simultaneous assessment of platelet aggregation and [Ca²⁺]_i signaling. Measurements are conducted in a cylindrical glass aggregometer cuvette (8-mm diameter) in a reconfigured dual-emission spectrofluorometer. The platelet suspension is stirred with a novel stirrer, comprising an opaque Teflon cylinder with a bar magnet at its base and a UV-grade methacrylate stirring vane at its top. This stirrer protrudes into the excitation beam of the spectrofluorometer to ensure that platelet aggregates cannot settle below its detection zone.17 Platelet aggregation is monitored through measurement of transmitted light intensity, with the lower part of the transmitted light beam blocked so that the stirrer does not interfere with these measurements. Platelet [Ca²⁺]_i is monitored through fluorescence measurements perpendicular to the excitation beam. Reflections from the curved surfaces of the cuvette are eliminated by use of a vertical polarizer in the excitation beam and a horizontal one in the emission beam. This arrangement provides efficient stirring of the platelet suspension and ensures that the fluorescence signal is representative of the entire population of platelets regardless of the extent of aggregation.