A rho-related protein, which was ADP-ribosylated by exoenzyme C3 from Clostridium botulinum, was found in both cytosol and membrane fractions of the testes of adult rats. This protein was observed in the cytosol fraction of the testes already in the newborn, did not significantly change in level up to about 7 weeks, and thereafter became clearly reduced. In the membrane fraction, in contrast, the protein was not found in the newborn rats. These observations suggest that there are some relationships between the maturation of the testes and changes in the subcellular distribution of this rho-related protein.