Millions of children throughout the world suffer from some form of nutritional insufficiency resulting- in a retardation of their physical and mental development. Recently results of behavioural studies have suggested that inclusion of a programme of psycho-social stimulation in the management and treatment of such children might cause enhancements in their mental development. However, the morphological effects of a period of sensory stimulation on neural development in previously undernourished subjects have largely remained uninvestigated. Experiments described in this Thesis were designed to attempt such an investigation using the laboratory rat as the animal model. Rats were undernourished during their brain growth spurt period and then raised either in enriched or isolated environmental conditions. The enriched environmental condition consisted of a group of twelve male rats living together in a large cage containing a set of stimulus objects called "toys". The isolated environmental condition consisted of a single rat living in a small opaque cage which did not contain any toys. A parallel set of well-fed rats was raised in identical environmental conditions. The experiments were carried out on four separate occasions. At the end of the period of environmental modification various gross and microscopic characteristics of the brain were analyzed. These analyses included measurements of brain weights, forebrain dimensions and cortical depths. Stereo- logical procedures at the light microscopic level were used to estimate nuclear diameters and numerical densities of neurons and glial cells as well as neuronal perikarya.1 volumes in the visual cortex. At the electron microscopic level synaptic disc diameters, synaptic numerical densities and synapse-to- neuron ratios were estimated in visual cortical layers II and III. Results of these analyses suggested that environmental enrichment caused significant increases in forebrain weights and lengths as well as deficits in neuronal numerical densities particularly in the upper third of the visual cortex. It also produced significant increases in synaptic disc diameters and synapse-to-neuron ratios in cortical layers II and III. These changes were seen both in well-fed and previously undernourished rats Measurements of cortical depths and estimates of neuronal nuclear diameters, perikaryal volumes and glial cell numerical densities did not show consistent effects of environmental treatment. This latter finding is in contrast to several previously published results. Possible reasons for this are discussed. Results of the two-way analysis of variance tests on data combined from both nutritional groups indicated that the interaction between nutrition and environment was not significant for any of the measurements carried out. This suggested that the cerebral changes seen in both nutritional groups were similar in magnitude as well as direction. These results provide experimental evidence for the morphological basis of the rehabilitative potentials of sensory stimulation for previously undernourished subjects. This may have implications for designing a programme of therapy for children who have suffered an episode of earlylife malnutrition.