ABSTRACT A high incidence of atrioventricular septal defect was induced in rat fetuses by administration of N, N -his (dichloroacetyl)-1, 8-octamethylenediamine, bisdiamine, to pregnant Donryu rats. Induction of atrioventricular septal defect was highly dependent on day of bisdiamine administration. When bisdiamine was administered singly on day 9.4 of gestation or consecutively on days 9 and 10 of gestation, the incidence of atrioventricular septal defect in fetuses increased. Survival rate decreased with increase in the amount of bisdiamine administered. Normalized incidence 5 for atrioventricular septal defect, which was calculated as a product of incidence and survival rate, was as high as 54.5% when 175 mg bisdiamine was administered on day 9.4 of gestation. Atrioventricular septal defect thus formed was of the complete type associated with either persistent truncus arteriosus or tetralogy of Fallot, indicating that the conotruncal portion was unexceptionally malformed. The present model is useful for analysis of development and pathogenesis of atrioventricular septal defect and also contributes to clarifying the normal development of atrioventricular valves and septa.