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We have examined the effect of retinoic acid (RA) on axial pattern formation during bud development of the ascidian, Polyandrocarpa misakiensis. A bead containing various concentrations of RA was implanted into the distal portion of a bud at a site where morphogenic events do not normally occur. Control buds were implanted with beads containing dimethyl sulfoxide (DMSO), the solvent of RA. No apparent effect was observed in these buds containing beads treated with DMSO. In contrast, beads containing 100 μg/ml of RA could induce ectopic structures in the distal portion of buds in about 30% of the cases. The resulting animals had completely duplicated antero-posterior axes. Histological studies showed that, within two days of RA treatment, atrial epithelial cells situated just beneath the implanted bead became thickened and formed a gut rudiment that resembled the posterior structure of the animal. The effect of RA treatment was dose-dependent. The minimum concentration of RA required to induce a secondary axis was 100 ng/ml. Beads containing 1 mg/ml of RA had a lethal effect on the cells that surrounded the beads. These results are discussed in relation to the role of RA in axis formation and the mechanism by which positional values are specified during normal and aberrant bud development in ascidians.