We have introduced a chimeric plasmid, pLTR2TK, containing the mouse mammary tumor virus (MTV) long terminal repeat (LTR) linked to the herpes simplex virus type 1 thymidine kinase gene into the mouse germ line by microinjection. In one mouse line, the thymidine kinase gene is appropriately expressed in the lactating mammary glands of heterozygous females; expression also occurs in the ovaries of these mice. In heterozygous males of this line, and in a male derived from another microinjection, transcription of these new germ line genes occurs in the testes and is specifically stimulated by glucocorticoid hormones. Thus, the MTV LTR may contain elements that allow its expression both in lactating mammary glands and in testicular tissue.