Mouse oocytes and zygotes are semitransparent and large cells approximately 80 μm in diameter. Bisection is one of the easiest ways for performing micromanipulations on such cells. It allows living sister halves or smaller fragments to be obtained, which can be cultured and observed for long periods of time. Bisection can be used for different kinds of experiments such as analysis of nucleo-cytoplasmic interactions, the relationship between different cellular structures or between different parts of embryos, eventually for analyzing the developmental potential of embryonic fragments. Oocyte or embryo halves can be examined by immunostaining, by measuring different cellular functions and by Western blot and genetic analysis (e.g., RT-PCR). Here we describe a detailed protocol for the free-hand bisection of mouse zona pellucida-free oocytes and embryos on an agar layer using a glass needle.