Secretory component (SC) was measured in amniotic fluid, fetal serum, and maternal serum and compared with SC production during in vitro culture of uterine tissue segments from pregnant rats. The concentrations of SC in amniotic fluid did not change between days 14 and 20 of pregnancy. Similarly, there was no change in maternal or fetal serum during pregnancy, although, the levels of SC in sera were consistently higher than those in amniotic fluid. When uterine segments were incubated in vitro, release of SC was greater in the absence of cycloheximide than in the presence of cycloheximide at all stages of pregnancy. In contrast to SC values in amniotic fluid, however, SC production by uterine tissue changed markedly during pregnancy. SC levels were low during early pregnancy (day 7 post coitus) and increased to levels found in non-pregnant diestrous rats just prior to parturition (day 20). The findings suggest that the endocrine balance during pregnancy may play a central role in regulation of the uterus immune system. The pattern of SC release may reflect a need both to ensure protection of the fetus from the IgA immune system in early pregnancy and to prevent maternal infection during parturition by reactivation of this system.