Amino acids are the essential substrates for fetal growth and catabolism. The fetus is dependent on the placenta for the provision of amino acids, the first step being concentration of amino acids within the syncytiotrophoblast for subsequent transfer to the fetus. A reliable technique for the isolation of human syncytiotrophoblast plasma membrane has been described, and the suitability of this preparation for the study of amino acid uptake and membrane transport has been well documented. Using this technique, the microvillous vesicle uptake of alpha-aminoisobutyric acid (AIB), a nonmetabolizable amino acid, has been determined over multiple time points for normal (NL) pregnancies and those complicated by pregnancy-induced hypertension (PIH), non-insulin-dependent diabetes mellitus (NIDDM) and those delivering small-for-gestational-age (SGA) neonates. There was no significant difference in AIB uptake between NL pregnancies and those complicated by PIH or NIDDM. Compared to each of the above, AIB uptake by the SGA group was significantly less at each time point. These results suggest that normal placental amino acid transport mechanisms may be altered in SGA pregnancies. If so, such alterations may interfere with the normal provision of nutrients to the fetus and ultimately contribute to impaired growth in utero.