AbstractGonadal protein patterns were studied during development in the rat by two‐dimensional micro‐gel electrophoresis. Specific proteins were detected in both the male and the female sex at the morphologically indifferent state (two female‐ and one male‐specific) and during differentiation. At the onset of gonadal differentiation (day 14) two additional sex‐specific proteins were discovered in the male and two in the female. These proteins remained expressed during further development. One testicular protein was restricted to the cytosol of the tunica albuginea. The other one was absent from the tunica. In the female gonad, the two proteins were membrane‐specific, one present in germ cells, the other in somatic cells. In the testis, one additional protein was discovered at postnatal day 1. Thus according to biochemical criteria there is no indifferent state of gonadal development. The testis and ovary express sex‐specific genes both before and after the onset of gonadal di