In this study, we have examined the transfection of aqueous CdS quantum dots (QDs) in the cytoplasm of PC12 neuronal cells using polyethylenimine (PEI) as carrier. The CdS QDs were prepared using a unique aqueous synthesis method, at 5 nm in size and capped with 3-mercaptopropyltrimethoxysilane (MPS). They exhibited a quantum yield of 7.5% and a zeta potential of −25 mV. With PEI they formed complexes by electrostatic attraction. At PEI/QD number ratios of>100, the PEI–QD complexes obtained exhibited a saturated size of about 24 nm and a zeta potential of about 15 mV. Confocal microscopy showed that PEI–QD complexes of a PEI/QD number ratio of 200 were successfully internalized and uniformly distributed inside the cells, indicating that the PEI–QD complexes were able to rupture the vesicles to enter the cytoplasm without aggregation. In addition, we showed that the presence of the PEI did not reduce the photoluminescence of the QDs and only mildly reduced the mitochondrial activity of the transfected cells—with no apparent reduction at a PEI/QD ratio of <40 to about 30% reduction at a PEI/QD number ratio of 200.