1. The objective of the present study was to examine the reversibility of the effect of neonatal malnutrition on in vivo neuronal protein synthesis. 2. Neonatal malnutrition was achieved by letting the pups suckle mothers fed a low casein diet (6.4%) which decreased the amount of milk produced. The control group was composed of pups from mothers fed a 17% casein diet and kept on the same diet after weaning. A group of pups was undernourished during the suckling period, as above, but fed a 17% casein diet after weaning (recovered group). All animals were killed at 60 days of age. 3. Neuronal cell bodies were separated from forebrain by mechanical disruption of the tissue and centrifugation in a sucrose gradient. 4. Protein biosynthesis was measured by the incorporation of [14C]-leucine into proteins of neuronal cell bodies, after correcting for the specific activity of the precursor. 5. Forebrain cell number and cell size were evaluated by DNA content and the protein/DNA ratio, respectively. 6. The forebrains of the recovered group weighed less than those of the controls, perhaps due to a lower number of cells, which exhibited an average size similar to that of the control group. 7. The rate of protein biosynthesis was similar in malnourished and recovered groups, but both were only 57% of the rate of the control group. 8. We conclude that neonatal malnutrition promotes an irreversible adverse effect on in vivo neuronal protein synthesis by rats.