Fusion genes containing 1600 or 2000 base pairs of the bovine thyroglobulin gene 5' flanking region and the chloramphenicol acetyltransferase (CAT) coding sequence were constructed and used to generate transgenic mice. Altogether, 24 independent transgenic lines were obtained, and the expression of the transgene was assayed by measuring the CAT activity in different tissues. Depending on the transgenic lines, the fusion gene was either silent in all tissues or specifically expressed in the thyroid. The level of expression was found to be highly variable from one line to another and to be regulated by thyrotropin in a manner similar to the natural thyroglobulin gene. The methylation status of the integrated DNA was tested by digestion of DNA extracted from thyroid and other tissues with the isochizomers Msp I and Hpa II. It was found that one of the Hpa II sites was demethylated specifically in the thyroid.