Hypertrophy, defined as an increase in cell size without an increase in cell number, occurs in a number of conditions, including compensatory renal growth, diabetes mellitus, protein feeding, chronic metabolic acidosis, and chronic potassium deficiency. In vitro cell culture studies have been used to characterize the mechanisms involved in the development of hypertrophy. Two mechanisms have been identified and characterized. One mechanism involves regulation of processes that are also associated with the initial events of the hyperplastic growth process, and is referred as a cell cycle-dependent mechanism. The other mechanism occurs independently of these particular cell cycle processes, but involves regulation of protein degradation by lysosomal enzymes. This latter mechanism is referred to as a cell cycle-independent mechanism. In vivo studies suggest that both compensatory renal hypertrophy following uninephrectomy and diabetes mellitus-induced hypertrophy involve the cell cycle-dependent mechanism.