Abstract We determined the location of alloantibody formation in the allogeneically pregnant rat. Greater than 95% of the alloantibody-forming cells were found in the spleen. A few were found in the paraaortic, renal, and parathymic lymph nodes. None were found in other lymphomyeloid organs. In addition, in vitro alloantibody synthesis was largely confined to splenic cells. We conclude that the alloantibody response in the allogeneically pregnant rat is largely peripheral to the uterus and to the lymph nodes that drain the uterus. Therefore, the alloantibody response in the allogeneically pregnant rat cannot be considered an example of a local immune response. In addition, we confirmed that the anti-paternal humoral response against RT1Aa caused by pregnancies by DA males differs dramatically from conventional and secondary pregnancy-induced alloimmunizations because the hierarchy of the number of alloplaques that are developed by these four methods of detecting alloplaques is different for primary pregnancy-induced responses as compared with conventional or secondary pregnancy-induced alloimmunizations.