Studies have been made on the content of immunoreactive insulin (IRI) in the blood serum of 10--16-day chick embryos. A totally homologous radioimmune assay system consisting of a standard and labelled chick insulin together with antiinsulin guinea pig serum to chick insulin, was used. The results obtained show that IRI content increases with the age of embryos. Intravascular injection of glucose increases IRI content in embryos of all the ages investigated.