Crosses between two species of the rodent genus Peromyscus produce defects in both growth and development. The defects are pronounced in the hybrid placentas. Peromyscuys maniculatus (strain BW) females mated to P. polionotus (strain PO) males produce placentas half the size of the parental species, as well as growth-retarded embryos. In contrast, PO females mated to BW males result in defective conceptuses that display embryonic and placental overgrowth. These ‘parent-of-origin’-dependent phenotypes are consistent with previous studies that demonstrated altered expression of imprinted genes and genetic linkage of the overgrowth phenotypes to imprinted domains. In the present study, we take a broader approach in assessing perturbations in hybrid placental gene expression through the use of Mus musculus cDNA microarrays. In verifying classes of genes identified in microarray screens differentially regulated during hybrid placental development, we focused on those influencing the cell cycle and extracellular matrix (ECM). Our work suggests that cell cycle regulators at the G₁/S phase check-point are downregulated in the large hybrid placenta, whereas the small hybrid placenta is more variable. The ECM genes are typically downstream targets of cell cycle regulation and their misregulation is consistent with many of the dysmorphic phenotypes. Thus, these data suggest imbalances in proliferation and differentiation in hybrid placentation.