A model has been proposed to account for many of the intracellular actions of insulin. This model is called the insulin transduction system and consists of coordinated stimulation by insulin of the sodium pump and the Na:H exchange system in the plasma membrane. The primary end result of stimulation of the insulin transduction system is an elevation of intracellular pH, pHi. This model is a biophysical system which depends upon thermodynamic gradients across the plasma membrane and thus is a property only of the intact cell. Insulin is one of several agents that produce mitogenesis, that is, stimulate DNA synthesis and division in cultured cells. Arguments are presented that mitogenesis is triggered by activation of the insulin transduction system with a resulting elevation of PHi.