This study examines the effect of premature delivery on the switch from fetal (α2β2) to adult (α2β2) Hb, in lambs in which premature parturition was induced by the intrafetal infusion of ACTH or corticotropin releasing hormone. Of 10 chronically cannulated ovine fetuses given ACTH at the rate of 79 ng/min for a 15 min period every 2 h starting at days 125 (n = 9) or 126 (n = 1) of gestation, three died in utero at days 131, 131, and 130, respectively. The remaining seven were born alive at 133 ±1.6 days. Five control fetuses, treated with vehicle (0.9% NaCl, wt/vol) only were delivered at 149 ± 2.7 days of gestation, which is not significantly different from the duration of pregnancy in other chronically cannulated lambs in this flock. Hb switching, as measured by the globin synthesis ratio, β/α, was complete at term in the control lambs. The β/α globin synthesis ratio of the prematurely delivered lambs was not accelerated by birth, and was similar to that of control fetuses of the same gestational age still in utero. The four lambs surviving premature birth more than 70 days did not complete the switch to normal adult Hb until after 220 days postconception.