To characterize the placental activity for the transport of amino acids, the uptake of L-alanine was investigated by rapid membrane filtration using brush border membrane vesicles separated from the human placenta of early pregnant (12-13 gestational weeks) and late pregnant (37-38 gestational weeks) women. The uptake of L-alanine into the brush border membrane vesicles at early and late stage of gestation showed a pattern of transport dependent on the intra- and extravesicular Na+ concentration gradient (extravesicular Na+ greater than intravesicular Na+). The Na+ concentration gradient-dependent uptake of L-alanine into the brush border membrane vesicles at early and late stage of gestation also showed a dependency on the potential difference of the internal and external membrane. The transport of L-alanine into the brush border membrane vesicles was markedly augmented at late stage of gestation. On the basis of the double reciprocal plotting of the L-alanine concentration and the Na(+)-dependent uptake of L-alanine into the vesicles, Km and Vmax were calculated as parameters of the Na(+)-dependent uptake of L-alanine into the vesicles. In the early stage of gestation Km was 0.78 mM, and in the late stage of gestation was 0.80 mM. In the early stage of gestation Vmax (nmol/mg protein/20 sec) was 0.62, and in the late stage of gestation was 3.53. From the results, it was considered that the placental active transport mechanism of L-alanine is the same in the early and late stages of gestation, and it was shown that its transport activity increases greatly in late stages compared to the early stages of gestation.