AbstractThis study examines the use of heparin‐plasma as a culture medium for mammalian postimplantation whole‐embryo culture. The growth and differentiation of head‐fold rat embryo explants over 48 hours in a standard serum medium was compared with development of same stage explants over 48 hours in a plasma medium prepared using sodium heparin.Heparin disrupted the morphological differentiation of embryos, in a concentration‐dependent manner, from 25 μg sodium heparin/ml media (i.e., 5 IU/ml media), with overall embryo growth being adversely affected from a concentration of 200 μg sodium heparin/ml media (i.e., 40 IU/ml media). Defects of cranial neural tube development were the first apparent structural anomalies resulting from culture in heparin media. Forebrain development was grossly abnormal and associated with failure of eye development. As the heparin concentration in media increased, the cephalic neural folds remained widely open and the edges became increasingly everted, although differentiation of the heart, otic primordia, and pharyngeal arch persisted. Similar concentration‐dependent dysmorphogenic effects were seen when embryos were cultured in the standard serum media with added heparin.A minimum heparin concentration of 100 μg sodium heparin/ml media (i.e., 20 IU/ml media) was required to effectively inhibit coagulation of the plasma medium over the 48 hour culture period. Although embryonic growth was not adversely affected at this heparin concentration, morphological differentiation was severely disrupted. Therefore, heparin is not a suitable anticoagulant for the preparation of plasma for use in postimplantation whole‐