The mammalian neocortex consists of a mosaic of columnar units whose development is poorly understood. Optical recordings of brain slices labeled with the fluorescent calcium indicator fura-2 revealed that the neonatal rat cortex was partitioned into distinct domains of spontaneously coactive neurons. In tangential slices, these domains were 50 to 120 micrometers in diameter; in coronal slices they spanned several cortical layers and resembled columns found in the adult cortex. In developing somatosensory cortex, domains were smaller than, and distinct from, the barrels, which represent sensory input from a single vibrissa. The neurons within each domain were coupled by gap junctions. Thus, nonsynaptic communication during cortical development defines discrete multicellular patterns that could presage adult functional architecture.