The mechanisms for ascorbic acid transport were investigated in samples of human placenta obtained from normal term pregnancies. Results suggest that at very high ascorbic acid concentrations this vitamin may cross the placenta by simple diffusion. However, at lower ascorbic acid concentrations the predominant form of transport is carrier mediated and energy dependent. The energy used in the transport process can be derived from either glycolysis or oxidative phosphorylation. In addition, the transport system also requires the presence of sodium and of Na+-K+-dependent ATPase activity.