Suramin (250 mg/kg) and sodium aurothiomalate (100 mg/kg) both induced congenital malformations in the offspring following treatment of pregnant rats at either 8.5 or 9.5 days of gestation. Conceptuses from 9.5-day pregnant rats were cultured for 48 h in homologous serum to which either suramin or sodium aurothiomalate was added for the final 6 h. The presence of suramin up to 5 mg/ml had no effect on the protein content of yolk sacs at harvesting, but at 10 mg/ml caused a significant decrease. In contrast sodium aurothiomalate increased the protein content of yolk sacs at harvesting, in a concentration-dependent manner up to 100 micrograms/ml. Neither suramin nor sodium aurothiomalate significantly affected embryo protein content. When 125I-labelled polyvinylpyrrolidone was added to the culture serum for the final 6 h of culture, radioactivity was found in the yolk sac at harvesting, but not in the embryo. When suramin (2-10 mg/ml) was present for the final 6 h of culture, the quantity of radioactivity measured in the yolk sac at harvesting was significantly decreased in a concentration-dependent manner. No radioactivity was detected in the embryos. Sodium aurothiomalate had no effect on the uptake of 125I-labelled polyvinylpyrrolidone. When rat serum whose proteins were labelled with [3H]leucine was used as culture medium, radioactivity was found in the conceptus (both yolk sac and embryo) at harvesting. Suramin (5 mg/ml), present for the final or penultimate 6 h, significantly decreased the uptake of radioactivity into conceptuses and caused a significant increase in the proportion of the captured radiolabel that was associated with the yolk sac. Sodium aurothiomalate (25 or 500 micrograms/ml) had no effect on the total uptake of radio-label but caused a significant increase in the proportion of total radioactivity captured that was associated with the yolk sac. These data indicate that suramin, by interfering with both the uptake and intralysosomal digestion of protein, and sodium aurothiomalate, by inhibiting digestion of captured protein, disturb the normal pathway of yolk sac-mediated protein utilization with a consequent diminution of the supply of amino acids to the conceptus. The effects of suramin are seen only at high concentration, those of sodium aurothiomalate at much lower concentrations. It is likely that the two drugs exert their teratogenic action by their effects on the yolk sac nutritional pathway with resultant amino acid deprivation of the conceptus at a critical stage of development.