The X zone, innermost cortex of the mouse adrenal, was first represented by Kiyoshi Masui and Yasushige Tamura in the Japanese in 1924 and in the English in 1926. All the mice used by them were reared by themselves at the Laboratory of Veterinary Anatomy, Faculty of Agriculture, Imperial University of Tokyo. They were the first scientists in Japan that attempted to breed mice in their own laboratory and used them for scientific researches in the literature. The importance of the breeding of laboratory animals by users was first pointed out by Sahachiro Hata in 1914, who found Salvarsan as a specific medicine against Spirochaeta pallida in cooperation with Paul Ehrlich in 1910. Hata's idea on the breeding of laboratory animals by researchers themselves might inspire Masui. Hata himself tried to breed laboratory mice in the Kitasato Institute. They were called "French mice", because they were believed at that time to be introduced from Europe before 1910. Masui and Seitaro Adachi (1926) also used French mice in their basic studies. They compared the body size between French and Nanking mice with special attention to the genetic factors controlling the body size for the purpose of improving of army horses. They noticed that French mice were larger in body size and more active in reproduction. Hata noticed a difference in reaction to Salvarsan between two mice. Our survey of the literature revealed that the X zone in the mice used by Masui and Tamura had many distinctive characteristics. The major two found in 90-day-old virgins were the extremely thick X zone and its division into two sublayers, one being composed of vacuolated cells only and the other of non-vacuolated cells only. It seems most reasonable to postulate that the first representation of the X zone was made in French mice bred by Masui and Tamura themselves. These characteristics of the X zone in the mice used by Hata or Masui and Tamura were very similar to those observed in inbred strains of mice, KK and NC, which Kyoji Kondo established from Japanese fancy mice, Kasukabe and Nishikinezumi groups, respectively. The same characteristics were also found in 4 of 28 recombinant inbred (RI) strains between A/J and SM/J established by Masahiko Nishimura. Since both A/J and SM/J have not such characteristics, the thick X zone consisting of 2 sublayers with and without vacuolation seems to appear on a certain specific genetic background.(ABSTRACT TRUNCATED AT 400 WORDS)