Morphometric measurements have been made on rat sympathetic neurons at ages between 6 and 24 months. In neurons of the coeliac-superior mesenteric ganglion there is a marked decrease in the neuronal packing density between 12 and 18 months which is accompanied by increases in the size of the neurons and their nuclei. In the superior cervical ganglion, no changes in packing density are seen until 18 months after which a decrease occurs, accompanied by slight increases in the neuronal parameters. These post-maturation changes occurring throughout adult life reveal a continued dynamism of sympathetic neurons into old age as well as revealing further differences between populations of sympathetic neurons.