Amino acid transport System A has previously been demonstrated in apical membranes derived from rat placenta, as well as in apical and basal membranes derived from human placenta. We have studied Na+-dependent α-(methylamino)isobutyric acid (MeAIB) transport in apical and basal predominant membrane fractions prepared from 14 and 20 day gestation rat placenta. Marker enzyme recoveries did not differ significantly between age groups. Markers for intracellular organelles were also found to be comparable. Na+-dependent MeAIB transport was not sensitive to freezing and could be found in all membrane components tested. Kinetic parameters were studied-K(m) = 852 ± 215 μM, V(max) = 718 ± 126 pmol/5 sec/mg protein-20 day apical; K(m) = 748 ± 269 μM, V(max) = 610 ± 176 pmol/5 sec/mg protein-20 day basal-predominant; K(m) 614 ± 261 μM, V(max) = 123 ± 45 pmol/5 sec/mg protein-14 day apical. Kinetic parameters could not be determined in the 14 day gestation basal-predominant fraction because of the small amount of uptake present. We conclude that System A like activity is found in both apical and basal predominant membrane fractions derived from rat placenta, and that this activity increases over the last one third of gestation.