The in vitro antiplatelet aggregating activity of magnesium and magnesium associated with soluble citroflavonoids (hesperidin and eriodictin, 1:1) is well-established. The degree of inhibition of in vitro platelet aggregation activity produced by different concentrations of magnesium halogenates was determined. ADP (4 microM) was used to induce aggregation following Cardinal and Flowers' (1) technique. Antithrombotic activity was studied in vivo. The difference in duration of ADP-induced respiratory dysfunction was compared between animals fed 25 mg/kg magnesium halogenates for 10 days before testing and controls. An increase in circulating platelets was observed in rats treated with magnesium halogenates.