In expts. with 247 inbred mice it was found that treatment with testosterone-propionate over short periods of time caused X-zone disappearance in normal adolescent [male][male] and [female][female], adult [female][female], castrate adult [male][male] and adolescent and adult ovariectomized [female][female]. X-zone formation was prevented in young [male] and [female] mice. The time required to cause exptl. disappearance of the X-zone was found to correspond to the periods required for normal disappearance.