Reaggregated cell cultures from dissociated 7-day-old chick embryo whole brains were prepared, and the developmental profiles of acetylcholinesterase and choline acetyltransferase, in the aggregates, determined over a 30-day period. Enzyme activities in vitro, at different times of culture, typically lie between 30 and 60% of the values obtained for embryos or chicks of the same developmental age, up to day-10 posthatching. The increase in acetylcholinesterase activity over a 24-day period of culture/incubation is fourfold in the aggregates vs. sixfold for embryos, while the choline acetyltransferase values increase, during the same period of time, 32-fold in the aggregates vs. 17-fold in vivo. Choline acetyltransferase activity seems to be more dependent on good cell-to-cell contact than acetylcholinesterase activity. On the other hand, morphological studies on the aggregates with light and electron microscopy reveal a number of structural features characteristic of well-developed nervous tissue. It is suggested that aggregate cultures of chick brain cells are an adequate model system that is especially useful in analyzing developmental phenomena requiring free tridimensional interaction.