For luteal support therapy with progesterone for luteal phase defects such as infertility, sterility, in vitro fertilization and embryo transfer, it is important to have both a rapid rise in the plasma concentration and constancy. The additive effect of sodium caprate for mixed type progesterone suppositories, consisting of Witepsol W35 and Witepsol E85 at 1 : 1, was therefore investigated in vitro and in vivo.The strength, the thermodynamic characteristics, the surface structures, the drug release properties and the drug absorption properties were examined for these suppositories. The strength of the suppositories increased with the addition of sodium caprate. The addition of sodium caprate, however, had no effect on the thermodynamic characteristics of these formulas. The drug release from the progesterone suppositories occurred via the matrix and pores, and was confirmed by in vitro dissolution profiles in which the amount of progesterone is proportional to the square root of time. The sustained plasma concentration of progesterone was examined after the vaginal administration of mixed type suppositories.The plasma concentration of progesterone at an initial phase after the administration of 2 % sodium caprate/ mixed suppository (formulation D) was found to be twice that of the mixed suppository (formulation A, no containing sodium caprate). Furthermore, the area under the plasma concentration-time curve from 0 to 6 hours for formulation D increased to 1.3 times that of formulation A. The findings of this study suggest that the mixed type progesterone suppositories with the additional of sodium caprate should improve the effectiveness of luteal support therapy.