During the first 3 days of postfertilization development, there is an increase in the uptake of guanine by preimplantation mouse embryos. The major increase occurs between the 2-and the 8-to 16-cell stages, and a maximum of 0.115 pmoles/hr/embryo is reached at the early blastocyst stage. Upon entry into the embryo, the guanine is rapidly converted to GMP by hypoxanthine-guanine phosphoribosyltransferase. The GMP is then further phosphorylated to GDP and GTP, and the major intracellular guanine metabolite is GTP. Despite the presence of high activities of guanine deaminase in the embryos, no xanthine or uric acid is found. This is presumably the result of conversion of any xanthine formed to nucleotides.