The expression of neurotrophin-3 messenger RNA was studied by in situ hybridization in rat muscle spindles from the first embryonic stages of their formation until their mature appearance in adult animals. The first expression of neurotrophin-3 messenger RNA in developing muscles was observed at E19 in the firstly formed intrafusal fiber, the nuclear bag2 fiber. High levels of neurotrophin messenger RNA were found in the equatorial region of these intrafusal fibers in thin lines of cytoplasma around and between the packed-up nuclei. From E21 on, neurotrophin-3 messenger RNA was also present in the nuclear bag1 type intrafusal fiber. The expression of neurotrophin-3 messenger RNA in nuclear chain fibers, which were found in muscle spindles from day 6 after birth, was low and insignificant in comparison to the expression in the nuclear bag fibers. After completion of muscle spindle formation around the third week after birth, high levels of neurotrophin-3 messenger RNA remained present in the intrafusal fibers throughout life. During the entire period of muscle formation, examined from E15 on, as well as in mature muscles, no neurotrophin-3 messenger RNA could be detected in extrafusal fibers by in situ hybridization. The exclusive intramuscular expression of neurotrophin-3 messenger RNA in intrafusal fibers during development as well as in mature stages suggests the involvement of neurotrophin-3 in the formation and the maintenance of muscle spindles.