The paper presents the results of complex petrologic-geophysical investigations into the structure and petrography of deep levels of the Khibina and Lovozero plutons on the Kola Peninsula. Analysis of materials obtained by deep drilling and the results of three-dimensional density modeling suggest that lower levels of the plutons bear significant volumes of rocks that are not exposed at the modern erosion level. It was determined that the plutons contain continuous rock series typical of alkaline-ultrabasic massifs of the province: peridotite, pyroxenite, melilitolite, melteigite, ijolite, and carbonatite. Rocks of the alkaline-ultrabasic series account for no less than 30 vol % of the Khibina and 25 vol % of the Lovozero Massif within the 12.5-km-depth level accessible for gravimetric studies. The agpaitic nepheline syenites in the cores of both massifs were determined to include large bodies of miaskitic syenites. Synthesis of geologic-geophysical and petrologic-geochemical data led us to develop a model for the origin of the two complexes as a result of the synchronous evolution of mantle sources, which were situated at various depths, but had a common system of magma conduits and produced both basanite-phonolitic and foiditic melts.