In two gilts, following doses of either 47.7 or 64.6 mumol progesterone, a secondary increase in steroid concentration in portal and mesenteric blood plasma occurred 35-40 min after dosing or 10-15 min after an intravenous (i.v.) dose of cholecystokinin. It is suggested that this increase is splanchnic blood progesterone concentration indicates that the steroid undergoes enterohepatic circulation. In five ovariectomized gilts given single i.v. doses of 47.7 mumol progesterone, the steroid was cleared from the circulating plasma bi-exponentially; the mean half-lives (t 1/2) of the two components were 2.47 (+/- 0.36) and 33.5 (+/- 6.63) min. The t 1/2 of the first component was compatible with the liver being a principal organ of clearance.