T4, T3, and rT3 were determined by RIA on daily serum samples collected from five catheterized fetal sheep during the 2-week period before spontaneous vaginal delivery (PTD). Samples were also available from three newborn lambs during the first 4 h of life. T3 concentrations began to increase 4-6 days before spontaneous vaginal delivery, from a stable geometric mean concentration of 30 ng/dl to a mean concentration of 131 ng/dl on the day of delivery (day 0). T4 concentrations decreased in four of the animals and remained unchanged in one during this time interval. rT3 concentrations decreased from a stable mean of 472 to 249 ng/dl on day 0. The prenatal increase in serum T3 levels correlated temporally with increasing fetal serum cortisol concentrations. The mean T3 concentration increased further after delivery to a level of 487 ng/dl at 1 h of age without significant changes in T4 or rT3 levels. The following conclusions were reached: 1) fetal serum T3 concentrations increase while T4 and rT3 concentrations decrease during the 4-6 days preceding spontaneous labor, and 2) T3 concentrations increase further after delivery. Available evidence suggests that the prenatal increase in fetal serum T3 levels is mediated by a cortisol-induced increase in T4 to T3 conversion in fetal tissues. The mechanism(s) of the postnatal increase in serum T3 concentrations in the newborn is not clear.