Previous light microscopic investigations have indicated that, in the rat, the innervation of mesenteric lymph nodes may be less dense than that of axillary nodes. However, nerves of the enteric system are difficult to visualise by light microscopy. Therefore we quantified the density of innervation of axillary and mesenteric lymph nodes at the ultrastructural level. The results show a highly significant difference in the density of innervation between these 2 groups of lymph nodes, but morphologically the type of the innervation does not seem to differ. In previous studies, nerves were found predominantly in regions characterised by aggregations of plasma cells. In view of this association, we suggest that the difference in innervation may reflect the migration of plasma cells out of mesenteric nodes and into the mucosa of the gut wall. By contrast, in peripherally located nodes, plasma cells tend to remain within the lymph nodes, and hence the density of innervation of these nodes is greater.