It has been suggested that the principal output cells of the main olfactory bulb, the mitral cells, along with the glomerulus they enter, form an anatomical and functional column. To test the extent to which mitral cell somata lying close together in the mitral cell layer are connected to the same glomerulus, we reconstructed 267 mitral cells labeled by extracellular injections of horseradish peroxidase (HRP) in the external plexiform layer. Results show that apical dendrites tilt rostrally from the somata to the glomeruli and this tilt is gradually greater as the somata are located more and more rostrally in the olfactory bulb. The apical dendrites of most mitral cells in the same region of the bulb are parallel. We analyzed the degree to which dendrites were parallel by measuring the difference in angle of all pairs of apical dendrites in each section, with the restriction that no cells were separated by more than 500 microns. About half of these pairs of dendrites differed in angle by 20 degrees or less and were therefore said to be parallel. The degree of parallelism did not vary with the cell pair location or with intercell distance. Study of glomerular connections of pairs of mitral cells as a function of intercell distance revealed that 96% of mitral cells connected to the same glomerulus were separated by less than 120 microns, while 72% of cells connected to adjacent glomeruli and only 29% of cells connected to distant, i.e., nonadjacent, glomeruli were separated by less than 120 microns.(ABSTRACT TRUNCATED AT 250 WORDS)