During periimplantation, the estrogen biosynthetic capability of the porcine conceptus is transient, coincident with morphological development (trophoblast elongation), and temporally associated with maximal uterine release of insulin-like growth factor-I (IGF-I) into the luminal fluid. We investigated the possible linkages of conceptus steroidogenesis and IGF-I by characterizing the developmental expression of the pig conceptus cytochrome P450 aromatase (P450arom) and cytochrome P450 17 alpha-hydroxylase (P45017 alpha) genes during periimplantation (days 8, 10, 12, 15, and 18) and the potential regulation by IGF-I of conceptus P450arom messenger RNA (mRNA) content. Primer pairs derived from porcine P450arom and P45017 alpha DNA sequences were used in reverse transcription-polymerase chain reaction analysis of conceptuses of differing morphologies and developmental stages. RNA transcripts for P450arom and P45017 alpha were readily detected only in conceptuses obtained at day 12 of pregnancy, with RNA levels varying according to morphological stage. P45017 alpha gene expression was maximal in spherical conceptuses greater than 5 mm in diameter, whereas P450arom mRNA was expressed in small (3-5 mm) and large (6-10 mm) spherical (large >> small), as well as day 12, filamentous conceptuses. Using primers derived from the human IGF-I receptor (IGF-IR) DNA sequence, constitutive expression of IGF-IR mRNA in porcine conceptuses throughout early pregnancy was demonstrated. Constitutively low expression of conceptus RNA transcripts for IGF-I ligand was similarly observed. However, uterine luminal fluid concentrations of IGF-I on day 12 were highly correlated with conceptus morphology, with maximal IGF-I in uteri possessing filamentous conceptuses. In vitro addition of IGF-I to day 12 conceptuses of filamentous or spherical morphologies resulted in distinct effects on P450arom mRNA abundance. Filamentous conceptuses had increased amounts of P450arom mRNA when treated with IGF-I. In contrast, IGF-I decreased the levels of P450arom mRNA in spherical conceptuses. IGF-I treatment did not alter mRNA expression of the IGF-IR gene in conceptuses of either morphology. These results demonstrate that transient expression of P450arom and P45017 alpha genes in perimplantation porcine conceptuses, document the constitutive expression of the conceptus IGF-IR gene, and support the involvement of endometrial IGF-I in modulation of embryonic P450arom mRNA abundance.