The purpose of these experiments was to compare the effects of vitamin A on developing and regenerating limbs in Xenopus laevis tadpoles. Each tadpole had one hindlimb amputated to induce regeneration while the contralateral developing limb was left intact. Tadpoles at stages 50 through 54 were treated by immersion in retinol palmitate at doses ranging from 0.3 to 75 i.u. ml-1, for periods ranging from 1 to 14 days. Developing limbs usually became hypomorphic as a result of the treatment, with results varying with stage and treatment from slight phalange derangements to total disruption of pattern, or complete inhibition of limb development. Regenerating limbs gave a variety of responses including hypomorphic regeneration, proximodistal or anteroposterior duplication of skeletal elements, or complete suppression of regeneration. The response to retinol palmitate of developing limbs was clearly different from regenerating limbs. Hypotheses which might explain the results were discussed and a hypothesis which proposed a dual action of vitamin A affecting both the cell cycle and the mechanism of pattern regulation was proposed.