The relationship between growth and pattern specification during development remains elusive. Some molecules known to function as growth factors are also potent agents of pattern formation. This raises the possibility that growth factors could act in pattern formation via an effect on the cell cycle. We have tested the significance of the length of the cell cycle for gene expression and pattern formation in developing chick limb buds by locally slowing the cell cycle. When anterior cell cycles are lengthened by reversible inhibition of DNA replication or by other means, some genes characteristic of the posterior polarizing region are expressed, and digit duplication is observed. Conversely, when posterior cell cycles are slowed, expression of some posterior-specific genes is inhibited, but the pattern is normal. These results indicate that control of the length of the cell cycle could play a primary role in pattern formation by influencing the complement of genes expressed in a particular region of the embryo.