The effects of a 30 d period of undernutrition, followed in some animals by nutritional rehabilitation, on neuronal connectivity in adult rat cerebellum were investigated using the disector method. There was no significant difference between well fed (719+/-74, mean+/-S.E.) and undernourished (709+/-53) synapse-to-neuron ratios in 134-d-old rat cerebellar cortex, nor was there a significant difference in synapse-to-neuron ratios between control animals (941+/-71) and previously undernourished rats (813+/-42) at 175 d of age. However, the age-related changes were significant (P < 0.05) in the controls, but not in the experimental group. It may be that the period of undernutrition caused subtle changes in the rehabilitating group which reduced the capacity for growth seen in well fed, matched control animals.