The period length of the running wheel activity rhythm of cockroaches (Leucophaea maderae), which is lengthened by Li + , was found to be shortened by 0.7% by administering 50 mM RbCl in the drinking water. Replacing the Rb + solution by H 2 O for four weeks did not bring the period length back to the control value. This is probably due to the long biological half life of Rb + in the cells. The phenomenon is another instance in which the effect of Rb + is opposite to that of Li + .