In the testes of 47 embryos, fetuses and newborn rats (Fisher strain), until the 15th day of life, the appearance and numerical dynamics of "premyoid" and myoid cells were analysed by light microscopy. The material was prepared routinely (fixed in Bouin fluid, embedded in paraffin, and cut to the thickness of 7 micrometers). The embryos of the 12th to 17th day of gestation were serially sectioned in toto, while fetal testes on the 18th to 21th day of gestation were removed and serially cut as well as the rat testes after birth. In each testis ten seminiferous tubules (cords) sectioned perpendicularly were analysed and only in two testes was it possible to find out nine but not ten perpendicularly sectioned tubules. Until the first week after birth, the lengthened cells that envelope tubules (cords) are called "premyoid". In the second half of the second week two types of lengthened cells in the envelope could be seen: one with a very thin, dark nucleus and the other with a lighter and thicker nucleus located peripherally, toward the interstitium. The former are considered myoid cells and the latter as fibroblasts. The numerical dynamics of "premyoid" and myoid cells evidently depends on the "pulsatile" effect of gondotropins. In the prenatal period, it was possible to see two increases in the number of these cells: on the 16th and on the 20th day.(ABSTRACT TRUNCATED AT 250 WORDS)