Experiments were conducted to compare the in vitro development of different cerebellar cells in primary surface cultures to that observed in their co-cultures. Single cultures of embryonic (E.19) and postnatal (P 7) cerebellar cells as well as their co-cultures were established. Changes in cell-size distribution and in high-affinity GABA-uptake were studied during the first three weeks in the different cultures. Morphological analyses showed that in single cultures of embryonic (E 19) cells a number of large and middle-size neurons survived on the top of confluent monolayer of large flattened astroglial cells. In cultures of postnatal (P 7) cerebellar cells small tetanus toxin positive non-GABAergic neurons proved to be the most abundant cell-constituents while no confluent feeder-layer of non-neuronal cells was formed. In co-cultures both embryonic and postnatal morphological features were observed but we could not observe any improvement in either survival or maturation of the neuronal types studied. A transient increase in glial GABA-uptake, however, was observed in both postnatal and co-cultures soon after plating of postnatal cerebellar cells.