Background: In the recent days there has been an increase in diseases known as "angiogenic diseases" characterized by pathologic vascularisation. In the rat, the development of embryonic vessel starts to occur at 9.5 days of gestation. In mammals, the vascular system starts developing in a very early embryonic stage. The majority of rat embryo circulation system gets complete approximately at 11 - 12 days. Therefore the in vitro study of 9.5 - 11.5-day old embryo culture could be a suitable model to study the effects of angiogenic and antiangiogenic substances on yolk sac vascularisation. In the present study, the effects of Interleukin-12 (IL-12) on the yolk sac vascularisation are investigated during the in vitro embryo culture, where the latter angiogenic factor was added to serum.