In this study, we measured transfection efficiency in vitro and in vivo using the following nonviral approaches of gene delivery: injection of plasmid DNA, electroporation-assisted, liposome-enhanced, and integrin-targeted gene delivery, as well as the combination of these methods. Four histologically different tumor models were transfected with a plasmid encoding the green fluorescent protein (GFP) (B16 mouse melanoma, P22 rat carcinosarcoma, SaF mouse sarcoma, and T24 human bladder carcinoma) using adherent cells, dense cell suspensions, and solid tumors. Emphasis was placed on different electroporation conditions to optimise the duration and amplitude of the electric pulses, as well as on different DNA concentrations for effective gene delivery. In addition, transfection efficiency was correlated with cell density of the tumors. The major in vivo findings were: (a) electroporation-assisted gene delivery with plasmid DNA, employing long electric pulses with low amplitude, yielded significantly better GFP expression than short electric pulses with high amplitude; (b) electroporation combined with liposome-DNA complexes yielded the highest percentage of transfected tumor area in B16F1 tumor (6%); (c) transfection efficiency of electroporation-assisted plasmid DNA delivery was dependent on tumor type; (d) integrin-targeted vector, alone or combined with electroporation, was largely ineffective. In conclusion, our results demonstrate that some nonviral methods of gene delivery are feasible and efficient in transfecting solid tumors. Therefore, this makes nonviral methods attractive for further development.