The aim of this study was to investigate the changes in follistatin, an activin binding protein, during the oestrous cycle, gestation and parturition in ewes using a radio-immunoassay for total follistatin, which uses dissociating reagents to remove the interference of activin. Follistatin concentrations remained unchanged (2.7 ± 0.2 ng ml-1) during the oestrous cycle and decreased as pregnancy progressed. Follistatin concentrations in allantoic fluid also decreased during gestation, whereas in amniotic fluid follistatin concentrations reached a peak at day 75 of gestation (9.8 ng ml-1) and had decreased to 4.4 ng ml-1 at day 140. Follistatin concentrations in fetal blood (7.0 ± 0.5 ng ml-1) did not change from day 50 to day 140 of gestation but were significantly higher than in matched maternal samples (3.1 ± 0.3 ng ml-1). Circulating follistatin in ewes was significantly increased on the day of parturition (5.6 ± 0.6 ng ml-1) compared with the days before parturition (2.7 ± 0.4 ng ml-1), but had decreased by day 2 after birth. Blood samples from newborn lambs showed that plasma follistatin concentration (13.4 ± 2.3 ng ml-1) was significantly higher than that of the mothers and remained high for at least 7 days after birth. These data support previous studies of the human menstrual cycle indicating that follistatin is not an endocrine signal from the ovary; however, in contrast to human pregnancies, follistatin concentrations in sheep decreased and become high only after or during parturition. This difference observed between species may reflect different physiological effects of follistatin or may be the result of measurement of different isoforms.