The effects of splenectomy on luteolysis in pseudopregnant rabbits were observed. Pseudopregnancy was induced in the rabbits by mating with vasoligated mature males and injection of human chorionic gonadotrophin (D0). In these rabbits, the concentration of serum progesterone increased for 7 days after the induction of pseudopregnancy. In the control group (sham-operation), the concentration of serum progesterone returned to the level of the pre-ovulatory levels by 14 days of pseudopregnancy. On the other hand, in the splenectomized group, in which splenectomy was performed at D7, the serum progesterone concentration was maintained at the level of the functional luteal phase (D7) at least until 21 days of pseudopregnancy. These findings indicate that the spleen is concerned with luteolysis of the rabbit.