Growth in tissues and organs can be stimulated by either partial ablation or increased demands for physiological activity. Both causes appear to operate via identical pathways whereby the mounting requirements for extra functional output stimulate DNA replication and concomitantly promote specific protein synthesis. When certain representative organs are reduced in mass to varying extents, the time of onset and maximum response coincide within wide rangers of injury, but the magnitudes and durations of the responses are directly proportional to the intensities of the stimuli.