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Characterization of the spontaneous and evoked activity of patterned biological neural networks and discovery of the underlying mechanisms is a major step towards designing functional networks for biosensor, biocomputing and neural prosthetic applications. Here we report on the addition of bicuculline methiodide, a GABAA antagonist, to a low density network of embryonic hippocampal neurons linearly patterned on top of planar microelectrode arrays. Our investigation of the differences in the spontaneous and evoked activities before and after drug addition revealed two distinct behavioral trends. One group of electrodes (Group II) showed a marked increase in spontaneous spike rate, burst rate, and burst duration whereas the other (Group I) was unaffected. Likewise, the reliability of evoked activity was observed to be significantly lower in the presence of bicuculline if the stimulating electrode belonged to Group I as compared with Group II. We discuss how these findings might suggest the existence of cells at various stages of maturity in our culture system.