Insulin resistance in vivo and impaired insulin binding to isolated adipocytes are characteristic of diabetic ketoacidosis in the rat. To determine the respective roles of diabetes and acidaemia in the genesis of the binding defect, insulin binding to adipocytes from alkali-treated ketoacidotic diabetic and ammonium chloride acidotic rats was studied. Reversal of the acidaemia of ketoacidotic rats by sodium bicarbonate infusion (pH 6.73 +/- 0.027 to 7.35 +/- 0.027, p less than 0.001, n = 12) increased adipocyte insulin binding (0.51 +/- 0.21% to 2 x 10(5) cells/ml, n = 6 untreated versus 1.10 +/- 0.27% to 2 x 10(5) cell, n = 6 treated, p less than 0.05). Scatchard analysis showed this to be due to an increase in insulin receptor concentration. Ammonium chloride infusion caused marked metabolic acidaemia (pH 6.72 +/- 0.04, n = 12) and insulin binding to adipocytes was markedly decreased (0.81 +/- 0.12% to 2 x 10(5) cells/ml n = 6 versus 2.40 +/- 0.22% to 2 x 10(5) cells/ml, n = 6 in controls p less than 0.02), due to a change in receptor concentration. The apparent affinity of the receptor was markedly decreased in diabetic animals compared with normal controls but was unchanged in ammonium chloride acidotic animals. Thus in diabetic ketoacidosis there is both decreased affinity and number of insulin receptors partially reversible by prolonged alkali infusion. Only changes in affinity appeared to be specific for the diabetic state.