This paper describes the embryotoxicity and teratogenic effects of (2"R)-4'-O-Tetrahydropyranyladriamycin (THP). The drug was administered intravenously to female rats at 0.01, 0.03, 0.1 or 0.3 mg/kg daily from day 7 to day 17 of pregnancy and to female rabbits at 0.01, 0.05 or 0.1 mg/kg daily from day 6 to day 18 of pregnancy. Results were summarized as follows. Rats THP, at the highest dose of 0.3 mg/kg, decreased body weight gains of pregnant females. This dose caused a decrease in body weights of fetuses, tendencies to increase the rate of death of fetuses or of resorption, an increase in the number of lumbar vertebrae and a delayed ossification of forelimbs of fetuses. Other parameters were not affected by THP at any dose levels. At any dose levels, THP did not produce external, visceral or skeletal malformations in the offspring (F1), nor did it affect the development, physiological functions, behavior, mating, fertility or pregnancy of the offspring. However, at the highest dose level, THP decreased the weight of testes of the offspring. The results suggest that the maximum "no effect" dose level of THP to pregnant females and offspring is 0.1 mg/kg/day intravenously. Rabbits The highest dose of THP, 0.1 mg/kg, decreased the consumption of food and water by pregnant females, but at any dose levels, it did not affect their body weight gain. THP did not cause teratological effects such as external malformation or visceral and skeletal anomalies in the fetuses at any dose levels tested. The results suggest that the maximum "no effect" dose of THP is 0.05 mg/kg/day intravenously to pregnant females and above 0.1 mg/kg/day intravenously to fetuses.