Magnocellular neurosecretory neurons in the supraoptic nucleus were examined in aged and young subjects to assess the ultrastructural correlates of cellular activity including cell, nuclear and nucleolar size, as well as the percentage of the cell occupied by Golgi, mitochondrial and rough endoplasmic reticulum compartments. Morphometric analysis was performed on arbitrarily selected cell profiles using a computerized morphometry protocol. Approximately 50% of those neurons examined from aged subjects exhibited a statistically significant increase relative to the young rats in total cell area and all cellular organelles examined except nuclear area. Moreover, an increase in the number and density (per unit membrane area) of synaptic contacts onto those neurons was observed. These findings suggest an increase in synthetic activity by a subpopulation of supraoptic magnocellular neurons, and support the concept of neuronal plasticity in aged rats.