A pendulum can be stopped by a single impulse of the right magnitude, delivered at the proper time; started again, its phase will have changed. A biological oscillation, although it is a vastly more complicated phenomenon, can likewise be arrested by a single stimulus of a definite strength delivered at the proper time. This remarkable fact emerges from experiments I have conducted on two biological clocks, in organisms in the plant and animal kingdoms. The most interesting potential applications are, of course, those that involve the internal clocks of Man.