Pazzi conspiracy (pät´tsē), 1478, plot against Lorenzo de' Medici (Lorenzo il Magnifico) and his brother Giuliano, designed to end the hegemony of the Medici in the Florentine state and to enlarge papal territory. It was instigated by Pope Sixtus IV, his nephew Gerolamo Riario, Archbishop Salviati, and members of the Pazzi family, a wealthy Florentine family that rivaled the Medici. Actually, the Pazzi were tools in the conspiracy, which aimed not only at the death of the Medici, but at the elevation of Riario to power in Florence. Details of the plot were worked out by Salviati and the Pazzi while Riario and the pope remained in Rome. On Apr. 26, during High Mass at the cathedral, Giuliano de' Medici was stabbed to death, while Lorenzo escaped with a wound. The enraged Florentines seized and killed the conspirators. The Medici remained firmly entrenched in power.