Haematocrit values, plasma osmolality and the plasma concentrations of sodium, potassium, chloride and insulin were measured in carotid arterial blood before, during and after intravenous infusion of NaCl (0.5 mol 1-1) and KCl (0.5 mol 1-1) at 2 ml min-1 for 105 min into six conscious splenectomized sheep. Hypertonic NaCl infusion was associated with a fall in haematocrit of 1.30 +/- 0.10% (P less than 0.001) and no consistent change in plasma insulin concentration occurred during this infusion. Hypertonic KCl infusion caused the haematocrit to increase by 1.70 +/- 0.39% (P less than 0.001) and the plasma insulin concentration to increase by 60.0 +/- 16.3 mu U ml-1 (P less than 0.01). It was concluded that this increase in insulin concentration was caused by elevation of the plasma potassium concentration and was not due to coincident increases in plasma chloride concentration or osmolality. Shrinkage of the extracellular fluid volume during KCl infusion made no major contribution to the increase in insulin concentration which was probably the result of increased release from the pancreas.