To test whether pregnancy has any effect on amino acid metabolism, we examined in two experimental conditions (1) the effect of hyperinsulinemia on the blood concentration and net hepatic balance of amino acids, and (2) the effect of hyperaminoacidemia on the hepatic handling of amino acids. Experiments were performed in conscious virgin and pregnant rabbits after an 18-hour fast. In the first protocol (hyperinsulinemia), an increment in the plasma insulin level (approximately 45 and 20 microU/mL in the portal vein and artery, respectively) with euglycemia maintained causes a similar decrease (approximately 27% to 34%) in blood amino acid concentrations without any changes in the net hepatic uptake of amino acids in both groups of animals. The hepatic uptake of branched-chain amino acids (BCAA) was practically negligible, whereas there was a consistent uptake of gluconeogenic amino acids in pregnant and nonpregnant rabbits. In the second protocol, hyperaminoacidemia leads to a significantly lower increase in the net hepatic uptake of glycine and serine in pregnant rabbits as compared with nonpregnant rabbits. The same trend was observed for the uptake of individual BCAA, but it did not reach statistical significance. We conclude that in pregnant rabbits (1) insulin does not modify the hepatic uptake of amino acids, and its ability to suppress the release of amino acids from peripheral tissues does not seem to be affected when compared with that in nonpregnant animals, and (2) when hyperaminoacidemia occurs, a greater amount of gluconeogenic amino acids (glycine and serine) would escape the liver, suggesting a higher availability of these circulating amino acids for the fetus.