The yolk-sac is known to be a route for the transport of passive immunity from mother to fetus in the rat. The main purpose of the present paper is to describe an experimental system for ultrastructurally studying the kinetics of the uptake and transport of immunoglobulin by rat yolk-sac. This system has the advantage of enabling the membrane to be externalized and then exposed to protein under controlled environmental conditions whilst at the same time maintaining the conceptus in connection with the in situ placenta. Preliminary investigations have utilized homologous anti-horseradish peroxidase (HRP) IgG (detected as antibody by application of HRP) or HRP alone. Comparison has been made with the localization of endogenous IgG transmitted in vivo after immunization of the female rat with HRP. The results show the rapid binding of IgG to membrane since only 30 sec were sufficient for this attachment to occur. Moreover, the endocytic process also appears to be very fast as localization of IgG in clusters or patches, caveolae or pits and even rare microvesicles is observed within 8 min. On the other hand, no binding of HRP to microvilli was observed and, unlike IgG, HRP became located in the apico-tubulocanalicular system.