1. Purified anterior pituitary growth hormone has been given to pregnant rats and a study made of the cerebral development of the young in terms of behaviour, electrical activity of the brain and the quantitative histology of the cerebral cortex.
2. The experimental treatment resulted in an increase in the size of the young at birth. The maturation of innate and reflex behavioural responses was little affected but the performance of cortically mediated behaviour was enhanced. Little significant change was observed in the electroencephalogram other than an abnormal response to photic stimulation.
3. These physiological effects were associated with a modified pattern of cortical maturation consistent with a hypertrophy of neurones. This was reflected in an enlargement of the perikarya and an expansion of protoplasmic processes resulting in an increase in the statistical probability of interaction between neurones.
4. The results are discussed in relation to the earlier and somewhat dissimilar findings reported by Zamenhof (1942), and are regarded as consistent with previously formulated hypotheses linking the structure of the cerebral cortex with its mode of functioning.