Transplants creating presumptively excess or deficient proximodistal segments were performed to investigate pattern regulation in stage 21 and stage 23 chick limb buds. Excess skeletal elements were not removed by regulation. Deficiencies appeared to be regulated by host tissue. The ability of the host tissue to participate in regulation declined in more proximal tissue and with increased age. These results suggest that there are two requirements for pattern regulation. First, the tissue in question must be sufficiently close to the ectodermal ridge before it can respond. Results of previous work and implications for pattern formation are discussed with these requirements in mind.