Radiolabelled trichlorfon, dimethyl-(2,2,2-trichloro-1-hydroxyethyl)phosphonate, was administered by stomach tube to pregnant guinea pigs on days 37 and 52 of gestation and examined by the whole body autoradiography method. The results showed that the radiolabelled compound was rapidly distributed to the main organs of the dam with the highest concentrations in the liver, the kidney and the lung. No substantial concentration of radioactivity occurred in the fetuses until 30 min after administration. The uptake in the fetuses was more pronounced at the later gestational stage when the concentration in the fetal liver equalled that of the placenta.