Mammalian neural stem cells hold great promise for the treatment of central nervous system disorders. However, to be a viable clinical treatment for the millions of individuals afflicted with these disorders, it is necessary to develop cell expansion protocols. Although difficult to grow in bioreactors, neural stem cells can be expanded, in carefully designed media as aggregates of brain tissue. The objective of this study was to examine the control of the aggregate size in a batch culture by manipulating the agitation rate, and hence the liquid shear and oxygen transfer rate, in bioreactors. This is very important because large aggregates can develop necrotic centers of dead cells due to transport limitations of key nutrients. Manipulation of the agitation rate allowed us to control the average aggregate diameter to 150 mum, below levels where necrosis would occur. Moreover, for the best conditions, viable stem cell densities of 1.2 x 10(6) cells/mL were achieved in a batch culture with viabilities remaining above 80% for the majority of the runs.