Amino acid transport in the renal tubules of 21-day fetal offspring of protein-deprived (4% casein diet) and adequately fed (24% casein diet) rats was studied by measuring the in vitro accumulation of glycine and the amino acid analog, alpha-aminoisobutyric acid (AIB). The prenatally protein-deprived (PPD) fetuses were smaller than the controls with a mean weight of 3.8 g compared to 5.5 g, and their kidneys were 0.4% of body weight, compared to 0.5% of body weight in the control group. Assessment of the renal proximal tubule transport mechanisms indicated that they were capable of transporting glycine and AIB at day 21 of gestation. There were no differences between the two groups in total water content or extracellular fluid content of the renal cortical tissue slices. The samples from PPD fetuses were able to accumulate concentrations of amino acids equal to or greater than those of control fetuses. The accumulation of AIB per gram of tissue was almost twice as great in the PPD tissue slices as in the control slices, whereas the accumulation of glycine per gram of tissue was equal in the two groups. Comparison of the fluid content and extracellular space of the PPD and control tissues suggests that this difference in uptake was not due to changes in the fluid distribution. It is proposed that the increase in AIB absorption by PPD renal tissue is related to retardation in maturation of the tissue. Transport of glycine, which matures later in development, may be immature in both groups. The greater amino acid uptake may conserve amino acids and thus have survival value in PPD young.