AbstractThe uptake of14C‐AIB into uterine tissue and some other organs was studied during early implantation. Mice in experimentally delayed implantation were given estrogen and14C‐AIB at different time intervals. Mice in delayed implantation given only14C‐AIB displayed a low transport rate into the uterine tissue, comparable with that for the diaphragm muscle. However, if the estrogen injection preceeded that of the14C‐AIB by 4 to 8 h the transport capacity increased by a factor of at least 4 times while the uptake for the diaphragm muscle remained low. Under such conditions14C‐AIB‐accumulation into uterine tissue was favoured for at least 4 h ofin vivoincubation with the labeled