Fluctuations in the concentrations of cortisol and progesterone in fetal plasma, maternal plasma, and amniotic and allantoic fluids were measured in samples taken at 10-min intervals over a 90-min period from three groups of sheep sampled at different times during late pregnancy. During the last 30 days of gestation there was a significant rise in the mean concentration of cortisol in fetal plasma and amniotic fluid and a significant correlation between the cortisol concentration in these two fluids. The concentration of cortisol in allantoic fluid exceeded that in amniotic fluid. The concentration of cortisol in fetal plasma varied in a pulsatile manner, however the coefficient of variation (CV) within animals was greater (36%) on days -11 to -20, relative to the day of parturition (day 0), than on days -21 to -30 or days -5 to 0(15-19%). The CV values for cortisol in amniotic fluid and maternal plasma during the last 30 days of pregnancy were 20-50% and two at five times greater than the intraassay CV. The concentration of progesterone in amniotic fluid increased after day -20 but was not correlated with that in maternal plasma or fetal plasma. The concentrations of progesterone in paired samples of amniotic fluid and allantoic fluid were similar. The CV values for progesterone (18-34%) were similar in fetal and maternal plasma and amniotic fluid and did not change significantly during late pregnancy. Changes in the concentration of progesterone were unrelated to changes in cortisol. Interpretation of steroid profiles in fetal plasma and fluids through late pregnancy should take into account these short-term fluctuations in hormone concentrations.