ExtractNormal and undernourished 3-week-old rats were examined for activity of crude mitochondrial acetylcholinesterase (a marker for nerve-ending particles) in four brain areas. Littermates of these animals were used for determination of four other brain enzymes which are known to increase in activity during rat brain growth. Significant deficits (in units per gram wet weight) in crude mitochondrial acetylcholinesterase activity were found in the forebrain (normal = 2085, malnourished = 1444), brainstem (normal = 709, malnourished = 536), and olfactory lobes (normal = 74, malnourished = 46) of undernourished animals. When enzyme activities were expressed in terms of tissue wet weight, whole brain butyrylcholinesterase, fumarate hydratase, and β-galactosidase were found to be unaffected by undernutrition, whereas 5'-nucleotidase activity was higher in undernourished (730 units/g dry weight) than in control animals (660 units/g dry weight). The results lend support to the hypothesis that those constituents of the brain which show a large increase in concentration during brain growth are those most likely to be affected by growth retardation in early life.SpeculationUndernutrition during the period of rat brain growth when interneuronal connectivity is being established may result in a deficit in the number of nerve-ending particles in the brain. This could imply that an impaired formation of synaptic connections may account for some of the functional changes which result from growth restriction at this time.