Abstract To determine the number of structural genes expressed in the oviducts of laying hens and hormone-withdrawn chicks, the complexity of the polysomal poly(A)-containing RNA population of both oviduct types has been analyzed by measuring the kinetics of hybridization of mRNA to its cDNA. The results obtained reveal the presence of approximately 13,000 diverse mRNA species in the oviducts of laying hens and hormone-withdrawn chicks. Heterologous hybridizations showed that most, if not all, of the mRNA species present in the oviducts of laying hens are also present in the oviducts of hormone-withdrawn chicks, but with altered frequencies. The mRNAs for the egg white proteins ovalbumin, ovomucoid and lysozyme have been purified, and cDNA has been synthesized from each mRNA. This cDNA has been used to determine the concentration of the three mRNAs in the oviducts of laying hens and hormone-withdrawn chicks. Ovalbumin mRNA, ovomucoid mRNA and lysozyme mRNA represent, respectively, 50.0, 6.6 and 3.4%—that is, 60%—of the total polysomal poly(A)-containing RNA in the oviducts of laying hens, but only 0.01, 0.0039 and 0.0046%—that is, 0.02%—of the total polysomal poly(A)-containing RNA of the hormone-withdrawn oviducts, a 3000 fold shift in relative frequency.