Mechanisms responsible for observed nonsphericity of supernova remnants are analysed. The following aspects are studied numerically: the influence of the initial nonhomogeneity of the distribution of circumstellar matter on the dynamics of ejecta (SN explosion happens in a disc-like envelope, produced by nonspherical mass loss from the presupernova); the dynamics of SN remnants moving in homogeneous gas (the formation of such remnant could be a result of the drastic decay of the neutron star in a binary system). The conclusion is drawn that the factor responsible for observed forms of SN remnants in the adiababtic stage of their evolution is nonhomogeneity of the circumstellar medium.