We describe disparities of cleavage rates in vitro vs in vivo using monkey embryos. Embryos were recovered from living monkeys by retrograde irrigation of the Fallopian tubes after spontaneous ovulation and normal fertilization in vivo. Cleavage rates of the embryos were studied in vitro, employing two culture media. The progression of embryonic development was compared with that in the natural setting (in vivo). The findings indicate that monkey embryos developing in vitro kept pace with the expected time course for only about 24 hours; indeed, extracorporeal conditions using either culture medium were insufficient to sustain normal cleavage rates thereafter. The asynchrony between cleavage rates in the two milieus may contribute substantially to perinidatory embryo wastage and subsequent implantation failure, even after successful in vitro fertilization.