Much interest in vertebrate embryology is now focused on early pattern formation in the frog, Xenopus laevis. In this species, the body plan is specified by a stable positional system set up by a cytoplasmic rotation in the zygote that occurs before first cleavage. Perturbation of this initial cellular event by a variety of means causes permanent distortions of the positional system. Until now it has not been possible to alter the positional system after it has been specified. However, we report here that lithium, when applied after specification of the body plan, can respecify the positional system of the Xenopus embryo such that dorsal, axial structures develop from cells that otherwise contribute to ventral structures. Lithium is usually considered to have negative effects on early embryo development, but our results show that lithium can act in a positive manner to produce structures which represent the uppermost values of the positional system. This discovery introduces a convenient means to study cellular and molecular mechanisms of early vertebrate pattern expression.