The majority of granule cells in lobule 1-8 of 12 day old, sagittally-sliced rat cerebella, die or fail to develop in the first 7 days of standard media cultures while Purkinje cells remain in large number. Previously, we demonstrated axonal growth and robust synaptogenesis at 14 DIV when the [K+]o was raised to 20 to 40 mM after 7 days of initial placement in standard [K+]o (Chen et al., SFN Abst 2003). Dissociated cell cultures of rat granule cells survived better in 25 mM [K+]o (Hacket al., 1993 ) and released transmitter (Cousins et al., 1997; Varming et al., 1997). In the present study, we asked if there are levels of [K+]o that further enhances circuit development in slice cultures maintained for extended periods. Slices were treated with 5, 10, 15 or 20 mM [K+]o either from the time of slicing to 14 days or after an initial 7 day period in 5 mM [K+]o. We were surprised to find that slices cultured, immediately, in 15 mM [K+]o had uniquely optimal granule layer survival in most lobules. This finding ...