Estrogens stimulate the rate of transcription of the ovalbumin and conalbumin gene in the chicken oviduct. The synthesis of ovalbumin and conalbumin mRNA was studied in isolated nuclei. RNA synthesized in vitro was distinguished from preexisting nuclear RNA by affinity labeling the in vitro products with a mercurated nucleotide and subsequent purification of the Hg-RNA on SH-agarose. The content of ovalbumin and conalbumin mRNA sequences in the in vitro transcripts was determined by hybridization to cDNA. After the withdrawal of implanted hormones from chickens, the synthesis of conalbumin and ovalbumin RNA increased 2.5- and at least 20-fold, respectively, by treatment with estrogens. The maximal rate of transcription of the conalbumin gene is achieved within 2 h after estrogen induction, whereas the rate of transcription of the ovalbumin gene becomes maximal after a lag of several hours. These results demonstrate that estrogens affect two genes in the same target cell differently.