The role of the pituitary gland in the regulation of the plasma concentrations of insulin-like growth factors (IGFs) in the late gestation sheep fetus has been examined. Singleton sheep fetuses were either hypophysectomized or shamoperated between days 110-120 of gestation. Blood samples were then collected via carotid cannulae at least three times weekly for the remainder of gestation. In some hypophysectomized fetuses T4 was administered (100 g/day) to overcome the hypothyroidism caused by hypophysectomy. Blood samples were also obtained from lambs during the perinatal period, neonatal lambs within 1-10 days after birth, and pregnant and nonpregnant adult ewes. All plasma samples were subjected to Sephadex G- 50 gel filtration under acidic conditions (pH 2.3) to eliminate IGF-binding protein activity. the fractions containing the free IGF peptides were collected and assayed for IGF-I by heterologous RIA, and IGF-II by a homologous RRA. Plasma concentrations of IGF-I and IGF-II did not change with advancing gestational age in any fetal group and were not affected by the prolonged gestation that results from hypophysectomy. the mean plasma IGF-I and IGF-II concentrations in the sham fetuses were 112 ± 8 and 1340 ± 112 ng/ml, respectively. Hypophysectomy without thyroid hormone replacement resulted in a significant decrease in plasma IGF-I concentrations to 50 ± 5 ng/ml, whereas IGF-II concentrations were not affected (1096 ± 124 ng/ml). IGF-I concentrations in the hypophysectomized fetuses that received T4 were significantly increased (67 ± 6.0 ng/ml) compared to those in the hypophysectomized fetuses that did not receive T4. the IGF-II concentrations in the hypophysectomized fetuses that received T4 were similar to those in the sham-operated fetuses (1120 ± 112 ng/ml). At term IGFI concentrations were increased (180 ± 21 ng/ml) and IGF-II concentrations were decreased (264 ± 25 ng/ml) compared to fetal values. Plasma IGF-I concentrations in the prepubertal lamb were similar to the fetal values. Pregnancy in the adult ewe was associated with a significant increase in IGF-II, but had no effect on IGF-I plasma concentrations. These data show that circulating IGF-I concentrations in the fetal lamb are under some pituitary and thyroid control, whereas IGF-II concentrations are independently of pituitary or thyroid status. We confirm, using a homologous assay, that fetal IGF-II concentrations are high and then decrease at term. These data also support the concept that a pregnancy-related factor may regulate plasma IGF-II concentrations. Fetal plasma IGF-I concentrations are not different from prepubertal or adult values, but are elevated at term. These data would also indicate that the pituitary-IGF-I axis is active during fetal life and may play a role in the pituitary-directed regulation of fetal growth.