The effect of undernutrition on the activity of some enzymes involved in purine metabolism, ie, adenosine aminohydrolase, adenylate aminohydrolase, and inosine phosphorylase in cerebral hemispheres, cerebellum, and brain stem of rats at different days of postnatal development was studied. Adenosine aminohydrolase, adenylate aminohydrolase, and inosine phosphorylase were assayed by radiochemical methods which involve the measurement of the radioactive products formed, ie, inosine, IMP, and hypoxanthine, respectively. The results obtained indicate that undernutrition affects the activity of the enzymes which control the concentration of purine nucleotides to a different degree in the three brain regions examined.