Concentrations of estrogen (E), progestin (P),and androgen (A) were determined in follicular fluid (FF), and concentrations of E and P were measured in peripheral plasma and perifollicular plasma during the estrous cycle in the pig. Also, concentrations of E, P,and A were determined in the utero-ovarian vein (U-OV) plasma every 2h during estrus period in pigs. It was observed that as follicles increased in size, the concentration of E, P, andA in the FF rises from 13,32 and 1 ng/ml at day 8 of the cycle to 180, 754, and 21 ng/ml at day 20 of the cycle, respectively. Approximately 12 h before ovulation there is a significant (P less than 0.01) drop of E, P, and A in the FF to 27, 120, and 0.77 ng/ml, respectivley. Concentrations of E and P in FF and peripheral plasma do not correlate during the second half of the estrous cycle. At day 20 of the cycle, E and P concentrations in the plasma were 20 pg/ml and 0.4 ng/ml, respectively. Concentration of P in the U-OV plasma started increasing 18 to 24 h after the onset of estrus, which is coincident with the drop of P concentration in the FF. As estrus progressed there was a linear drop of E concentration (r = 0.90) in the U-OV plasma. E and P concentrations in the FF were found to be about 1000 times greater than those in the peripheral blood and 5 times greater than in the blood obtained from the perifollicular capillary network at the end of the cycle.