Prostaglandin F 2α (PGF 2α) was measured by immunoassay in plasma and milk of four cows (six experiments). After 30 mg PGF 2α im, plasma PGF 2α peaked at 15 minutes (2.4 ± 0.7 ng/ml) and declined toward basal values by 3 hours; maximum milk PGF 2α (0.91 ± 0.12 ng/ml) occurred at 1 hour. The average excretion rate in milk was 2.9 μg/day 0.9 μg (0.003%) of which was due to the 30 mg PGF 2α injected. In six non-pregnant control cows, daily changes of milk PGF 2α and progesterone were not consistently related.