The effect of protein malnutrition (PM) on the embryotoxicity and teratogenicity of aspirin in rats has been studied. Protein malnourishment was produced from Days 7 to 21 of gestation by limiting the casein content of the diet to 5% in comparison to 20% for the normal animals. Six dose levels of aspirin (50-175 mg kg-1) were administered p.o. from Days 8 to 11 of pregnancy. Aspirin from 75 mg kg-1 d-1 induced growth retardation, embryolethality and teratogenicity, as shown by external, skeletal and visceral malformations. All these effects were potentiated by protein malnutrition.