Mouse and rat brain cells were dissociated by a simple mechanical sieving technique and studied in culture for the formation of aggregates and the activities of choline acetyltransferase, acetylcholinesterase, glutamic acid decarboxylase, tyrosine 3-monooxygenase, aromatic L-amino acid decarboxylase, catechol methyltransferase, and monoamine oxidase. Cells from fetal and neonatal tissue formed aggregates but not cells from tissue older than two days after birth. The pattern of development of enzyme activities in these aggregates varied with the age of starting tissue. The highest levels of specific activity for the neuron-specific enzymes were found after 3-4 weeks in culture for aggregates of cells derived from relatively undeveloped brains.