The synthesis of cellular proteins during early pregnancy was analyzed in mouse endometria incubated in vitro in the presence of [35S]-methionine. Endometrial proteins labelled on days 3 and 5 of pregnancy were compared with proteins from nonpregnant proestrus-phase endometrium. We have identified 23 proteins that were differentially expressed. These proteins were divided into four groups: (1)10 proteins synthesized most prominently in nonpregnant proestrus-phase endometrium that decrease or disappear in pregnancy; (2) 4 proteins that are synthesized only on day 3, i.e., in the immediate preimplantation period; (3) 3 proteins that appear only on day 5 coincidentally with the development of decidua; (4) 5 proteins that are synthesized by both day 3 and day 5 endometria but are not found in the nonpregnant endometrium. Hence, the synthesis of some endometrial cellular proteins increased and others decreased in early pregnancy. The function of these proteins is unknown. Four proteins (Mr 26, 51, 52 and 70 kDa), 2 of which are both cell associated and secreted (Mr 52 and 70 kd) were selectively synthesized in the preimplantation period and thus may mediate implantation.