Insulin binding to mouse oocytes and preimplantation embryos was assessed by light-microscopic autoradiography. Significant insulin binding was present on the cells of morulae and increased twofold at the blastocyst stage of development. Insulin binding was markedly decreased by native insulin and to a lesser extent by insulin-like growth factors (IGFs). No specific insulin binding was detected on oocytes or embryos throughout the eight-cell stage. Specific binding of IGF-I and IGF-II was also observed on the cells of blastocyst outgrowths. The findings demonstrate that specific binding of insulin and IGF is temporally expressed on the cells of pre- and peri-implantation mouse embryos. These results confirm and extend our previous immunofluorescence study.