The morphological differentiation of mitral cell dendrites during embryonic and early postnatal development was examined in the main olfactory bulb of rats to determine a possible role of afferent activity in the development of the dendrites. Mitral cells and olfactory nerve fibers were labeled with 1,1ะ-dioctadecyl-3,3,3ะ,3ะ-tetramethylindocarbocyanine perchlorate (DiI) and fluorescein-conjugated lectin (Ulex europeus agglutinin-I), respectively. Morphogenesis of mitral cell dendrites proceeded as previously described (Malun and Brunjes [1996] J. Comp. Neurol. 368:1-16); that is, undifferentiated dendrites with radial orientation were transformed into a single primary dendrite having a glomerular tuft and secondary dendrites extending tangentially into the external plexiform layer. Quantitative examinations in both pre-and postnatal rats revealed that the differentiation of primary dendrites, including tuft formation, increases in diameter and decreases in branching, started before birth, whereas differentiated secondary dendrites were only observed in postnatal animals. Mitral cells with more than two primary dendrites were found after embryonic day 21. The proportion of the mitral cells with differentiated dendrites increased postnatally. At postnatal day 10, almost all mitral cells had fully differentiated dendrites, and mitral cells with multiple primary dendrites were no longer seen. No significant change was found during development in the number of stem dendrites that arose directly from the cell body. Unilateral naris occlusion started on postnatal day 1 retarded differentiation of primary and secondary dendrites, and increased the proportion of mitral cells with multiple primary dendrites. These finding revealed that differentiation of mitral cell primary dendrites precedes that of secondary dendrites, and suggested that the differentiation of secondary dendrites proceeds in an activity-dependent manner.