1. Furazolidone (0·75 g/kg) will interrupt pregnancy in mice.
2. Foetal tissue is particularly sensitive to the drug at the time of or before implantation of the ovum.
3. Intra-amniotic injection of furazolidone in rabbits causes foetal death and resorption.
4. Furazolidone applied locally does not antagonize the proliferative effect of progesterone on rabbit endometrium.
5. The mode of action of furazolidone on pregnancy is discussed. It is suggested that it acts directly on the foetus.