AbstractMercuric chloride, methylmercuric chloride, and cadmium chloride directly affect the human placental syncytiotrophoblast microvillous membrane. These heavy metals alter the facilitated diffusion of α‐aminoisobutyric acid (AIB) into vesicles of this membrane in μM concentrations. Mercuric chloride abolishes temporal kinetics of AIB transport, inducing an initial increase in AIB transport (27% at 100 μM) but subsequently lowering equilibrium values when compared to equilibrium time points in control. Methylmercuric chloride and cadmium chloride inhibited the initial rate of AIB transport (40% and 21%, respectively, at 200 μM), but did not affect the equilibrium value of AIB transported when compared to equilibrium levels in control. These effects were concentration dependent. Methylmercuric chloride was more potent in inhibiting AIB transport than cadmium chloride. Methylmercuric chloride and cadmium chloride effects on AIB transport were observed with minimal preincubation with placental vesicles. However, preincubation was necessary for mercuric chloride‐induced perturbation of AIB transport. Cysteine protects against mercuric chloride‐ and methylmercuric chloride‐induced effects on AIB transport but did not reverse these perturbations. Mercury‐ and cadmium‐induced placental membrane toxicity result from interactions of these heavy metals with the placental p