A novel device for measuring platelet aggregation has been devised. The technique, which depends upon changes in electrical impedance caused by platelet accretion onto electrodes, is suitable for measurements of aggregation in either platelet-rich plasma (PRP) or blood. In PRP both turbidometric and electronic techniques give very similar responses to collagen, thrombin, prostaglandin endoperoxides, ADP, and arachidonic acid, although the electronic aggregometer gives no "shape-change" information and was somewhat more sensitive. In whole blood all these stimuli gave similar dose-related responses to those seen in PRP. The new technique is very suitable for investigating platelet pharmacology, since the inhibitors of aggregation, such as indomethacin and prostacyclin, can be conveniently quantitated in blood by using this technique.