The Kola Alkaline Province in the northeastern Fennoscandian Shield comprises the world's biggest agpaitic region consisting of the Khibina and Lovozero agpaitic complexes in addition to numerous carbonatite intrusions. Gravity data were used to create 3D models of the deep structure of these alkaline complexes down to the upper crustal level. Computer modelling was used for data analysis and presentation. The obtained data give strong evidence for the different internal structures of the Khibina and the Lovozero complexes. Both complexes at deeper levels are suggested to be composed not only of agpaitic nepheline syenites, but also of alkaline ultramafic rocks. The total volume of peridotite, foidite and melilitic rocks which form the lower zones of these two plutons range from 20 to 30% of their total volume. The Khibina and the Lovozero complexes have no common magma conduits within the uppermost crustal levels. Carbonatite intrusions of the Kola Peninsula form (i) subvertical lens-shaped igneous bodies, (ii) lopolith-like subsurface bodies with thin conduits, (iii) subvertical concentric bodies widening downwards which are suggested to represent the upper parts of large alkaline ultrabasic intrusions. The results support the idea of the uniform vertical zonality of carbonatite intrusions which may have had initially uniform magmatic reservoirs. Originally, the shape of the magma chambers for the carbonatite intrusions was close to a lens-like symmetric stock body 16-18 km in height with a vertical length/maximum diameter ratio close to 2:1. Differences in the on-surface structure of the carbonatite intrusions mostly depend on the level of erosion of the magma chambers. Comparative analysis of the morphology and internal structure of carbonatite intrusions have shown varying levels of erosion in different parts of the Kola Peninsula. This leads to the conclusion that the Precambrian crust, together with enclosed carbonatite intrusions, has undergone nonuniform erosion since the time of late Devonian alkaline magmatism. The southern part of the Kola basement appears to be the most eroded portion of northeastern Fennoscandia whereas the western and northwestern shield areas experienced less uplift since the time of the late Devonian alkaline magmatism.