Insulin, insulin-like growth factor-I (Igf-I), and insulin-like growth factor-II (Igf-II) are known to enhance growth in mouse preimplantation embryos. The addition of insulin, Igf-I, and Igf-II to mouse embryos in culture results in an increase in protein synthesis, cell number, and the proportion of embryos developing to the blastocyst stage. To study the role of the insulin-like growth factors in early human development, the timing of gene expression of insulin, IGF1, IGF2, and their receptors was analysed. Reverse transcription polymerase chain reaction (RT-PCR) was used to examine the presence of transcripts in preimplantation embryos. Following reverse transcription, strategically designed nested primers were used for amplification from cDNA. Transcripts for all three receptors (insulin receptor, IGF1R, IGF2R) were present in human oocytes and preimplantation embryos. However, of the ligands, only IGF2 transcripts were detected. This is consistent with expressed patterns seen in the mouse. As in the human, mouse Igf2 is the only ligand in the family expressed and has been shown to have an autocrine effect on preimplantation development. It has previously been shown that insulin and Igf-I are produced by the mouse maternal reproductive tract and have a paracrine effect on the preimplantation embryo. We speculate that a similar relationship exists in the human and that preimplantation development may be regulated by IGFs from both embryonic (IGF-II) and maternal (insulin and IGF-I) sources.