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While many metabolic effects of insulin on various organs and tissues have been demonstrated, the underlying mechanism of the hormone action has remained obscure. In the present paper evidence will be presented to show that insulin has an immediate electrochemical action on unwashed red cells. In subsequent papers an attempt will be made to relate this action to the hormone's sustained metabolic effects.The distinction between 'immediate electrochemical' and 'metabolic' effects is based on experimental conditions. The following points may be emphasized. All experiments to be described were performed at room temperature. The test systems were unwashed red cells suspended in simple inorganic saline media. No metabolic substrates-e.g. glucose, nucleosides-were added. Insulin and control solutions were introduced into parallel cell suspensions and immediate changes were recorded in three ways. First, extracellular pH and redox potentials (Eh) were monitored with electrodes. Secondly, the suspensions were spun without delay and electrolyte concentrations were estimated in the supernatant fluids and the lysed cell deposits. Third, where appropriate, manometric measurements were carried out in the Warburg apparatus. METHODSMaterial 'Unwashed red ceUls.' Fresh heparinized blood from fasting subjects was spun at approximately 1400 g for 15 min. The supernatant plasma and as much of the buffy coat as possible was removed. The spun cells were resuspended in the saline media indicated. No attempt was made to wash the cells free from plasma. The plasma content in the reconstituted extracellular fluids (measured as protein-nitrogen) varied between 2 and 5 % of the original plasma volume. Nucleated cell counts were < 500/cmm. Haematocrits were between 50-70%.

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