OBJECTIVE: To label rat neural stem cells (NSCs) with the complex of Sinerem, the ultrasmall superparamagnetic iron oxide (USPIO), and poly-L-lysine (PLL), and evaluate the feasibility of tracking the labeled cells with magnetic resonance imaging (MRI) in vitro and in vivo. METHODS: Sinerem was incubated with PLL to obtain the complex of Sinerem-PLL. The mesenchymal stem cells (MSCs) isolated from the bone marrow of SD rats were cultured and induced to differentiate into the neural stem cells. The second-passage cells were cultured overnight with the Sinerem-PLL complex, after which Prussian blue staining and transmission electron microscopy were performed to observe the nanoparticles in the cytoplasm. Cell apoptosis assay was performed to assess the cell viability 1 day, 1 week, and 2 weeks after the labeling. Cell tracking with 4.7 MR system was carried out in vivo and in vitro using T(2)WI and T(2)*WI sequences. RESULTS: The NSCs could be effectively labeled with Sinerem-PLL complex with the labeling efficiency exceeding 95%. Prussian blue staining showed numerous blue iron particles in the cytoplasm, and under transmission electron microscope, these particles accumulated in the endosomes/lysosomes. The labeling did not significantly affect the cell viability and proliferation. Remarkable low signal density changes of the labeled cells was seen on T(2)WI and T(2)*WI in vivo and in vitro. CONCLUSION: NSCs can be effectively labeled with Sinerem-PLL complex, and MRI can be used to track the labeled cells in vivo and in vitro.