The behavior of cells is generally considered to be regulated by environmental factors, but the molecules in the milieu of neural stem cells have been little studied. We found by immunohistochemistry that chondroitin sulfate (CS) existed in the surroundings of nestin-positive cells or neural stem/progenitor cells in the rat ventricular zone of the telencephalon at embryonic day 14. Brain-specific chondroitin sulfate proteoglycans (CSPGs), including neurocan, phosphacan/receptor-type protein-tyrosine phosphatase beta, and neuroglycan C, were detected in the ventricular zone. Neurospheres formed by cells from the fetal telencephalon also expressed these CSPGs and NG2 proteoglycan. To examine the structural features and functions of CS polysaccharides in the milieu of neural stem cells, we isolated and purified CS from embryonic day 14 telencephalons. The CS preparation consisted of two fractions differing in size and extent of sulfation: small CS polysaccharides with low sulfation and large CS polysaccharides with high sulfation. Interestingly, both CS polysaccharides and commercial preparations of dermatan sulfate CS-B and an E-type of highly sulfated CS promoted the fibroblast growth factor-2-mediated proliferation of neural stem/progenitor cells. None of these CS preparations promoted the epidermal growth factor-mediated neural stem cell proliferation. These results suggest that these CSPGs are involved in the proliferation of neural stem cells as a group of cell microenvironmental factors.