On the second day after the removal of a part of an adult rat's kidney mass, a mitotic wave is released among the tubule cells of the remaining kidney. This wave proceeds synchronously throughout the cortex and may be followed by a second but much smaller wave on the third day. The removal of increasing amounts of kidney tissue (a half, a whole or a kidney and a half) is paralleled by the release of mitotic waves of growing size. The peak of a larger wave occurs sooner after the resection than that of a smaller wave. The bisection of a kidney initiates more cell divisions than those expected by the diminution of kidney mass alone. This increase of the mitotic wave appears not to be caused by the presence of the transectional wound. As a working hypothesis it is proposed that the bisection leads to a relative increase in blood flow through the diminuted organ, thereby affecting a more effective drainage of growth‐inhibitor(s) and a corresponding enhancement of the mitotic wave. This latter is always preceded by a non‐mitotic period of slightly less than 30 hr. It is proposed that about the first 2/3 of this period may be taken for the inactivation of an organ‐specific growth inhibitor that suppresses the growth of the convoluted tubules of the intact adul