Holstein or crossbred beef cows were anaesthetized on Days 15 to 17 after oestrus; the ovarian artery (OA), ovarian (utero-ovarian) vein (OV) and a peripheral artery (PA), were catheterized for chronic blood sampling. Beginning on the day after surgery, 6 sequential blood samples were collected every 30-40 min twice daily from 8 cyclic and 6 pregnant cows during Days 16-20: 818 blood samples (including 216 OA and PA concurrent arterial pairs) were collected. Overall least squares means for PGF concentrations (pg/ml) in the OV, OA and PA of cyclic cows were 562, 228 and 106, respectively. A significant (P less than 0.01) OA-PA difference (122 pg/ml) suggests that a local transfer system, between uterine venous effluent and ovarian arterial affluent, is functional in the cow. A transfer efficiency of about 1% was estimated. In cyclic cows differences in OA-PA concentrations of PGF were minimal on Days 16-18 and increased to about 160 pg/ml during luteal regression (Days 19-20). In pregnant cows a biphasic OA-PA pattern of difference in PGF between days was detected, with a peak on Day 18 (136 pg/ml) which was not apparent on Days 19-20. Amplitude of PGF spikes in the OA was significantly higher in cyclic (725 pg/ml) than in pregnant cows (397 pg/ml). We suggest that pregnancy suppresses PGF delivery to the ovarian circulation, resulting in maintenance of the corpus luteum in pregnant cows.