Plasma amino acid pools show important variations throughout the gestational period in the rat, with decreased values at mid-pregnancy and recovered levels before parturition. This decrease in amino acid levels at mid-pregnancy is mainly due to changes in the gluconeogenic amino acid group. During pregnancy, whole tissue amino acid pools increase in the liver but no changes appear in other studied tissues. However, individual gluconeogenic amino acid concentrations change significantly in skeletal muscle, skin and liver through the gestational period. Pregnancy may be understood as a challenge to the mother's metabolism but the pattern of tissue amino acid changes is not similar to that found in some stressant situations and it should be the net consequence of maternal adaptations to the increased metabolic needs.