Penetration of pegylated (enveloped in polyethylene glycol) gold nanoparticles 5 and 30 nm in diameter through the placental barrier was studied in pregnant rats injected intravenously with these particles in a dose of about 0.8 mg Au/kg on day 10 of gestation. The particles were visualized in tissues by silver nitrate autometallography; the total content of gold in the fetuses was evaluated by atomic adsorption spectroscopy. Gold nanoparticles were detected in the fetal liver macrophages and in the spleens; high total content of gold in the fetuses was demonstrated for particles of both sizes. The data suggest that gold nanoparticles penetrate through rat placental barrier in vivo. No morphological changes were detected in the liver, kidneys, spleen, and brain of fetuses.