Changes in morphological dimensions of MDX mouse myofibres in M. rectus femoris were recorded after ballistic transfection (BT) with pHSADys and pVMMDys plasmids containing cDNA of the full-length human dystrophin gene. The dystrophin expression was observed by an immunomorphological procedure with P6 antibody and PAP method. Dystrophin positive (dyst+) myofibres were divided into two types, with a typical dystrophin expression under sarcoplasma membrane and an atypical expression through the whole sarcoplasm, respectively. The share of atypical dyst+ myofibres was seen to rise during the experiment from 27%, at 2-3 weeks after BT, up 84% by 2 months after BT. The atypical dyst+ myofibres usually underwent destruction. At the same time, the share of entire dyst+ myofibres decreased from 17 to 2-5% by 2 months. Morphological dimensions of the myofibres (square in mkm2, perimeter, smallest and largest diameters) were calculated with the help of computer analyser. The middle square of both types of dyst+ myofibres was larger than that of dyst- myofibres, both in BT target M. rectus femoris and in the same contralateral muscle, but never exceeded the value of middle square of C57B1 mouse myofibres in the same muscle. The form of dyst+ myofibres was not modified by the dystrophin expression. The nuclei of dyst+ myofibres remained in the central region of sarcoplasm. A conclusion is made that BT of human dystrophin gene inside MDX mouse myofibres allows dystrophin gene expression and enlargement of the dyst+ myofibres. Dystrophin expression is not able to induce a complete and stable differentiation of striated muscle of adult MDX mice.