Inadequate levels of circulating progesterone (P) seen in clinical practice can be increased by P supplementation. When supplementation is via the vaginal route, its presentation may be in many different suppository bases. We have measured circulating P in the follicular phase to compare delivery from the vagina after P was presented in three different bases (glycerinated gelatin, cocoa butter, and polyethylene glycol). The mean peak level achieved and the area under the curve were highest after supplementation from polyethylene glycol-based suppositories. The duration of elevation above baseline was similar with all three suppositories. It is concluded that the suppository base is important in controlling the amount of this steroid absorbed from a given amount presented by this route.