We studied the development of insulin binding to its receptor in chick embryos from days 2-18 of the 21-day incubation period. Using partially purified membrane preparations we found that insulin receptors on both brain and liver of day 18 embryos were typical insulin receptors by multiple criteria. Specific insulin binding to preparations of whole embryos was not detected on day 2, but was present by day 3. Insulin receptors were present on both heads and bodies of embryos by day 4. Binding to liver and brains increased with development between days 8 and 18; however, the patterns of increase in the two tissues were quite distinct. In liver, binding was low on day 8, but increased markedly by day 14 (P less than 0.01). Increases after day 14 were less impressive. In contrast, binding to brain preparations was relatively well established by day 8 and did not change significantly between days 8 and 14. An abrupt increase occurred between days 14 and 16 (P less than 0.01), and a second increase took place between days 16 and 18. We have previously shown that insulin is present in chick embryos before known pancreatic development. The current demonstration that insulin receptors are also present early in embryogenesis makes it possible that insulin may influence the embryo at early stages. In addition, the different patterns of increases in insulin binding in liver and brain during mid- to late development suggest complex and tissue-specific mechanisms of regulation.