Quantitative measurements of the major organelles in the mitral cell perikaryon as well as numbers of mitral somato-dendritic synapses, have been made on electron micrographs from rats aged 3 to 30 months. The volume fraction of the cisternae of rough endoplasmic reticulum (RER) is constant throughout the period studied. Hence the amount of RER per cell reflects changes in perikaryal size. Thus there is a 3-fold increase of volume of cisternae of RER per mitral cell from 3 to 27 months and a halving from 27 to 30 months. A similar pattern is seen for the volume of mitochondria per cell. The volumes of dense bodies and cisternae of the Golgi complex per cell show a different pattern, that of a linear increase throughout the period, with no suggestion of a decrease from 27 to 30 months. An interesting finding is that the volume fraction of ground substance (perikaryal cytoplasm exclusive of the measured organelles) shows a remarkably constant value from 3 to 27 months and then a highly significant decrease from 27 to 30 months. This decrease, restricted to the period from 27 to 30 months, suggests a fundamental breakdown in cellular homeostasis in the oldest animals. Numbers of somato-dendritic, mitral-to-granule synaptic junctions per mitral cell and per olfactory bulb show a significant increase from 3 to 24 and 27 months, respectively, and then a suggestive decrease from 24 and 27 to 30 months. This finding indicates that new synapse formation is possible in normal adult rats, perhaps even in quite old rats.