Uterine blood was sampled by venepuncture or an indwelling catheter in a total of 33 cyclic gilts and 26 mated animals subsequently confirmed to contain embryos; jugular blood was obtained simultaneously from catheterised animals. Prostaglandin F2 alpha and progesterone were determined by radioimmunoassay of the plasma. The concentration of PGF2 alpha in uterine venous blood of cyclic animals remained below 1.0 ng/ml until the corpora lutea were 12 days old. Highest PGF2 alpha values were associated with 15-17 day corpora lutea, with a mean of 5.9 ng/ml for six samples on Day 17. Likewise, the PGF2 alpha concentration in the uterine blood of mated animals did not exceed 1.0 ng/ml until the corpora lutea were older than 12 days, and a mean value of 6.0 ng/ml was found by acute sampling with 15-day corpora lutea. The highest mean concentrations of PGF2 alpha in uterine blood from a series of 14 catheterised pregnant animals were 2.8 and 2.3 ng/ml, respectively, with 15- and 16-day corpora lutea. Values for PGF2 alpha on the 17th, 18th and 19th days of pregnancy showed a downward trend. There was considerable day to day variation in the mean uterine and peripheral concentrations of progesterone in mated animals, but there was no sustained depression in response to elevated PGF2 alpha concentrations. The results suggest that exocrine secretion of PGF2 alpha into the uterine lumen of pigs under the influence of trophoblastic oestrogens does not provide a sufficient explanation for the establishment of the corpora lutea of pregnancy. Further attention should be devoted to the luteotrophic--as distinct from anti-luteolytic--rôle of pig conceptuses at the time of maternal recognition of pregnancy. Circumstantial evidence for luteal sensitivity to chorionic gonadotrophins is included.