Our studies the past 5 yr have concentrated on intracellular ice formation (IIF) in mature mouse oocytes at the metaphase stage of meiosis II. Here we report an analogous investigation of the temperature of intracellular ice nucleation in preimplantation embryo stages from one-cell to early morula suspended in 1 M ethylene glycol/PBS and cooled at 20°C/min to −70°C. Physical modeling indicates that oocytes and preimplantation embryos undergo very little osmotic shrinkage at that cooling rate. As a consequence, their interior becomes increasingly supercooled until the supercooling is abruptly terminated by IIF. Four categories of IIF were observed. The first two were 1) those undergoing IIF at temperatures well below the temperature of external ice formation (EIF; −7.2°C) vs. 2) those undergoing IIF within 1°C of the EIF temperature. The other two categories were those multicellular stages in which 3) all the blastomeres underwent IIF simultaneously vs. 4) those in which blastomeres underwent IIF sequentially. Embryos in categories 1 and 3 constituted the majority (80–90%), and for them, the mean IIF temperatures of one-cell, two-cell, four- to six-cell, and early eight-cell ranged from −37°C to −43°C, temperatures that indicate that IIF is a consequence of homogeneous nucleation. However, the IIF nucleation temperature of early morulae in categories 1 and 3 was markedly higher; namely, −23.1 ± 1.5°C. This marked rise in nucleation temperature coincides with the appearance of aquaporin 3 and gap junctions in early morulae (compacted eight-cell), and is presumably causally related.