Teratogenic effects of a single subcutaneous dose of ochratoxin A (1.75 mg/kg) on one of gestation days 4-10 has been studied in rats. Treatment on days 4, 5, 6 or 7 resulted in increased fetal resorptions, and decreased fetal weights. Fetal weights also were depressed after treatment on day 8. The highest number of resorptions, greatest depression of fetal weights and largest number of malformations occurred when ochratoxin A was injected on days 5, 6 or 7. External hydrocephaly, omphalocele, and anophthalmia were the major gross anomalies. Internal hydrocephaly and shift in position of esophagus were the main internal soft tissue defects. Major skeletal defects involved sternebrae, vertebrae and ribs. Subcutaneous injection of 1.75 mg/kg ochratoxin A seemed to be the minimum effective, single teratogenic dose. Lower doses (0.5 or 1.0 mg/kg) failed to produce any embryotoxicity or malformations and higher doses (e.g. 5.0 mg/kg) caused all fetuses to be resorbed.