Melatonin plays an important role in regulating the reproduction of seasonally propagated mammals. Its receptor MTNR1a (Melatonin receptor 1a) is mainly involved in circadian rhythm and reproductive regulation, and MTNR1b (Melatonin receptor 1b) is closely related to a variety of diseases. In order to investigate the biological function of melatonin receptor gene, the expression patterns and localization of MTNR1a and MTNR1b in different yak tissues were clarified in this study. The expression patterns of MTNR1a and MTNR1b in different tissues of adult male yak and in testes at different developmental stages (30 days old, 2, 4, 6 and 8 years old) was detected by qRT- PCR. Furthermore, the MTNR1a and MTNR1b in testes of yaks of different ages were localized by immunohistochemical technique. The results showed that the expression of MTNR1a mRNA was highest in pineal gland, followed by lung, muscle and testis. The expression of MTNR1a mRNA in the testes was gradually increasewith age, and tended to be stable after 4 years old. The MTNR1a protein is expressed in the testis tissue of yak at different developmental stages, and round spermatozoa showed strong immunolocalization, followed by primary spermatocytes. MTNR1a protein was expressed in pineal gland. The expression of MTNR1b mRNA was highest in pineal gland (P<0.05), followed by kidney, liver and hypothalamus. MTNR1b mRNA was expressed in yak testes of different ages, and the expression level increased with age, highest at 8 years old. The

immunohistochemistry result showed that the MTNR1b protein is mainly localized in round spermatids. The extensive expression of MTNR1a and MTNR1b genes in different tissues and testes of yaks at different developmental stages reveals their important roles in male yak reproduction and other physiological processes.