Two plutonium-metal critical assemblies have been studied at the Pajarito site in Los Alamos. Part I of this article describes Jezebel, the bare plutonium assembly, and gives its observed characteristics along with a few comparisons with enriched-uranium systems. Part II covers Popsy, a plutonium core in a thick normal-uranium reflector. As Popsy was relatively inflexible—intended only for a preliminary survey—its experimental program was much less complete than that of Jezebel.