Our purpose was to isolate microvillous plasma membrane from first-trimester placenta and to measure its transport properties with regard to Na+, H+, and a neutral amino acid. Microvillous membrane was isolated from first-trimester (10 to 13 weeks) and term (38 to 42 weeks) placenta and the purity determined. Uptake of 22Na+ was measured in the presence of an outwardly directly H+ gradient in the presence or absence of amiloride (0.5 mmol/L). The rate of dissipation of an H+ concentration gradient was determined with the H(+)-sensitive fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. 14C-methylaminoisobutyric acid uptake was measured in the presence and absence of an inwardly directed Na+ gradient. Purity, vesicle volume, vesicle orientation, and electron micrographic appearance of the first-trimester membranes were similar to those obtained from term placenta, but vesicle protein recovery was lower. Amiloride-sensitive Na+ uptake and Na(+)-dependent 14C-methylaminoisobutyric acid uptake was threefold to fourfold lower by first-trimester than by term membranes. The rate of H+ concentration gradient dissipation was greater in the former. The first-trimester microvillous membrane has similar general characteristics to that from term placenta, but its transport activity is quite different.