Recent studies have clearly shown that the expression of genes for gonadotropin-releasing hormone (GnRH) and its receptor in the rat reproductive organs including ovary, testis, placenta uterus and mammary gland. Moreover, luteinizing hormone (LH) classically known to be a main target product of GnRH in anterior pituitary has been found in rat gonads. These findings suggested the presence of local circuit composed of GnRH and LH in the rat gonads. The present study was undertaken to elucidate whether the genes for LH and its receptor are expressed in rat mammary gland. Expression of LH and its receptor genes in the rat mammary gland was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) and specific LH radioimmunoassay (RIA). The LH$eta$ transcripts in the mammary gland from cycling rats contained the pituitary type of LH$eta$ exons 1~3 encoding the entire LH$eta$ polypeptide but lacked the rat testis-specific LH$eta$ exon(s). Presence of $alpha$ -subunit transcripts in the rat mammary gland were determined by RT-PCR. The cDNA fragments encoding exons 2~7 of rat LH receptor transcripts were amplified in both rat ovary and mammary gland samples. We could detect the GnRH expression in mammary gland from cycling virgin rats, and this result disagreed with previous report that mammary GnRH expression is occured in lactating rats only. Considerable amounts of immunoreactive LH molecules with good RIA parallelism in standard curve were detected in crude extracts from the rat mammary gland, indicating that the immunoreactive LH materials in the gland might be identical to authentic pituitary LH. To our knowledge, the present study demonstrated for the first time the expression of LH subunits and LH receptor in the rat mammary gland. Our findings suggested that the mammary gland might be the novel source and target of LH and the mammary LH could be act as a local regulator with auto-and/or paracrine manner under the regulation of local GnRH.