The effect of splenectomy on the concentration of progesterone in pregnant wistar rats showing 4-day regular estrous cycles more than two cycles were investigated. The day after mating with the same male animal was designated day 0 of pregnancy. Splenectomy was conducted on day 0 (early-S), 6 (middle-S) and 13 (late-S) of pregnant rat, respectively. Blood sample were collected at day 1, 7, 14 and 21 of pregnancy. Concentraton of serum progesterone was determined by emzyme-linked immunosorbent assay (ELISA) using specific antibodies and emzyme conjugate(progesterone peroxidase labeled). In normal pregnant rats, serum progesterone concentrations were increased significantly (P<0.05) on day 7 and 14 of pregnancy compared with on day 1 of pregnancy and then dramatically decreased on day 21. However in early and middle splenectomized rats, progesterone concentrations were significantly (P<0.05) decreased on day 7 and 14 of pregnancy compared with same day of normal rats. These observations indicate that luteotropic action and steroidogenesis by corpus luteum of ovary may be regulated by splenocytes in rat.