A study was performed to determine the levels of estrogens and prostaglandin F2alpha in the stallion ejaculate. Simultaneous semen and blood plasma samples were collected from 19 stallions, 2 weeks apart, during the breeding season. Although not statistically different, the total mean estrogen content tended to be higher in seminal plasma (4447 pg/ml) than in blood (2497 pg/ml). A tendency was found for higher mean estrone sulphate concentrations than for total free steroid in both seminal (4116.1 vs 330.5 pg/ml) and blood plasma (2447.1 vs 49.5 pm/ml). Mean concentrations of estrone in ejaculate and blood plasma were 257.1+/-267.0 (SD) and 9.5+/-5.4 pg/ml, respectively. Estradiol-17beta concentrations were 73.4+/-87.4 and 40.0+/-27.6 pg/ml in ejaculates and blood plasma, respectively. Mean PGF2alpha concentrations tended to be much higher than total estrogens (1106.8+/-1636.4, SD, vs approximately 260 ng/ejaculate, respectively). To our knowledge this is the first report of PGF2alpha and estrogen concentrations in the stallion ejaculate.