Exposure of eggs of Xenopus laevis to a temperature of 1.0 degree C for 4 min or a pressure of 8000 psi for 5 min in a critical period before first cleavage results in embryos exhibiting a reduction and loss of structures of the body axis. The deficiencies occur in a craniocaudal progression which is dose dependent. In the extreme, totally axis-deficient embryos with radial symmetry are formed. Maximum sensitivity to cold and pressure occurs at 0.6 of the time from fertilization to first cleavage and extends from approximately 0.4 to 0.8, the period between pronuclear contact and mitosis, and the approximate period of gray crescent formation. The effects of cold and pressure resemble those previously reported for uv irradiation in that (1) the types of axis-deficient embryos produced are morphologically indistinguishable; (2) sensitivity in all cases ends before 0.8; (3) cold and uv effects, although not those of pressure, can be prevented by cotreatment with D2O; and (4) impaired eggs can be rescued by oblique orientation. We interpret these results as follows: during the 0.4-0.8 period the egg reorganizes its contents in a manner critical for subsequent development of the embryonic body axis. The reorganization process involves cytoskeletal elements, some of which are sensitive to cold, pressure, and uv, and protected by D2O. Rescue by oblique orientation can be understood as the result of a gravity-driven reorganization of the egg's contents, supplanting the normal mechanochemical process impaired in treated eggs.