It has been suggested, but not shown, that in the fetus placental lactogen (PL) may affect the regulation of the IGFs and fetal metabolism. To examine the effects of PL on the circulating concentrations of the IGFs, IGF-binding proteins (IGFBPs), glucose, free fatty acids (FFAs) and amino nitrogen (AN), we infused late gestation sheep fetuses with recombinant ovine PL (roPL). Five chronically-catheterised sheep fetuses were infused intravenously with three 24 h infusions of saline, roPL (100 micrograms bolus then 500 micrograms over 24 h) and then saline again. Fetal roPL infusion increased plasma oPL from 0.4 +/- 0.1 to 3.3 +/- 0.5 nM (mean +/- S.E.M.; P < 0.05; factorial analysis of variance and Scheffé's test). Fetal plasma IGF-I, IGF-II, insulin, FFAs and blood glucose were unaffected by the roPL infusion. Fetal plasma IGFBP-3, as measured by Western ligand blotting, decreased by 30% during fetal roPL infusion while other fetal plasma IGFBPs were unaffected. Fetal roPL infusion decreased fetal blood AN from 7.3 +/- 0.5 to 6.6 +/- 0.2 mM (P < 0.05). Maternal plasma IGF-I, IGF-II, IGFBPs, insulin, FFAs, blood glucose and AN were unaffected by the fetal roPL infusion. Saline infusion had no effect on any parameter. The data suggest that PL is not a significant determinant of plasma IGFs in the late gestation sheep fetus although there may be an indirect effect via alterations in levels of IGFBP-3. The effect of fetal roPL infusion on fetal blood AN concentrations may suggest some role for PL in the regulation of fetal amino acid metabolism.