Insulin-like growth factor-I and parathyroid hormone are both known regulators of bone formation. In this study, human recombinant IGF-I and bovine PTH (1–34) and their combination were studied for their effects in vitro on the proliferation of embryonic chick osteoblast-like cells (osteoblasts) and in vivo on bone formation in normal rats. Osteoblasts from 17-day-old chick embryos were cultured in serum-free BGJb medium containing 0.1% bovine albumin. After 2 days, IGF-I and/or PTH were added. Twenty-four hours later [3H]thymidine incorporation into trichloroacetic acid precipitable material was quantified as an index of cell proliferation. This has previously been shown to reflect actual cell division. IGF-I at doses ranging from 0.85 to 13.6 nmol/l caused a dose-dependent increase in [3H]thymidine incorporation into osteoblasts. PTH alone (10 to 1000 pmol/l) had no significant effect. However, when combined with IGF-I, PTH potentiated the mitogenic effect of IGF-I and achieved statistical significance at 30 and 100 pmol/l (p <0.05). This potentiation was also studied in vivo. The right hindlimbs of rats weighing 150 g were infused intra-arterially by an osmotic minipump with graded doses of IGF-I (0.1 to 0.4 nmol/day) and/or PTH (0.27 nmol/day) for 7 days. The rate of trabecular bone apposition (formation) was measured by double tetracycline labelling and compared with the contralateral uninfused limb which acted as the control. Histomorphometric data revaled that neither IGF-I nor PTH alone had a significant effect on trabecular bone apposition rate compared with control limbs. The co-infusion of IGF-I (0.4 nmol/day) and PTH (0.27 nmol/day) resulted in a marked increase in trabecular bone apposition rate. The results of 2 studies were significant at p < 0.01. These data suggest that PTH potentiates the effect of IGF-I on bone formation both in vivo and in vitro.