Neurons in the myenteric plexus of Auerbach of the jejunum, ileum, colon and rectum in 6- and 24-month-old rats have been visualised by an enzyme histochemical method in laminar preparations of the muscularis externa. Neuron somata numbers and sizes were recorded in the preparations. Significant reductions, of at least 40%, in the numbers of neurons in the ganglia of the myenteric plexus of the 24-month animals were recorded in all regions of the intestine, especially in the colon where the neuron number decreased by over 60%. These figures do not take into account changes that might have occurred in the overall length of the intestine during adult life. Measurements of neuron sizes suggest that the neuronal fall-out with age affects all categories of neuron equally and implies that all the intrinsic neuronal systems of the gut could be equally affected in old age.