Pregnant rats from day 14 of pregnancy and pups were fed a control diet (24% casein) or a deprived diet (8% casein) to obtain a control and a pre- and post-natal protein-deprived group. From 50 days of age, all groups were fed a balanced commercial stock diet for different periods. A significant reduction in body weight was observed in the perinatally protein-deprived group (PPD) after the different periods of nutritional recovery studied. Maximal binding capacity (MBC) and apparent affinity constant (Ka) of liver nuclear T3-receptors in the PPD group were similar to those in the control group. Mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosolic malic enzyme (ME) activity were significantly increased in the PPD group in all the periods of nutritional recovery studied, except in the 8 months old group where it was non-significant. Plasma T3 levels were higher in rats 3 and 15 days after the removal of the hypoprotein diet while it was normal after longer periods of nutritional recovery. Serum T4 was not modified in any group. The results indicate that protein undernutrition during perinatal life may induce an activation of hepatic T3-dependent enzymes which persists even after long periods of nutritional recovery. The lack of modification at the nuclear T3-receptor level, where supposedly the first T3-signal takes place, raises the possibility of an amplification of this signal at a step beyond the receptor.