Poor solubility of polycation complexes with DNA is one drawback for their in vivo use as gene delivery systems. PEGylation often can improve the solubility of the complexes, minimize their aggregation and reduce their interaction with proteins in the physiological fluid. We investigated in vivo application of polyethylene glycol (PEG) modified polyethylenimine (PEI) for gene expression in the central nervous system. Varied numbers of linear PEG (2 kDa) were grafted to branched PEI (25 kDa) from the average number of PEG per one PEI macromolecule at 1-14.5. While higher degrees of PEG grafting did not improve gene expression, a PEI conjugate with one segment of PEG was able to mediate transgene expression in the spinal cord up to 11-fold higher than PEI homopolymer after intrathecal administration of its DNA complexes into the lumbar spinal cord subarachnoid space. Improved gene expression with this conjugate was observed as well in the brain after the lumbar injection. As assessed in in vitro studies, the PEI conjugate with a low degree of PEG grafting was able to reduce the size of polymer DNA complexes, prevent the aggregation of complexes, decrease the interactions of the complexes with serum proteins, counter the inhibition of serum to gene transfer, and enhance transfection efficiency, although not significant in affecting complex formation and reducing in vitro cell toxicity of PEI. The study provides the in vivo evidence that an appropriate degree of PEG modification is decisive in improving gene transfer mediated by PEGylated polymers.