Intracellular pH regulation was studied in papillary muscle from STZ-induced diabetic rat hearts. In control bicarbonate solution there was no difference between the steady-state pHi values recorded from diabetic or normal papillary muscle. The addition of insulin had no effect on the pHi of either group. The amplitude of NH4+-induced alkalinization and the time course of recovery from alkalinization were similar in both normal and diabetic muscles. In both preparations, the recovery from alkalinization was similarly delayed by the disulfonic stilbene DIDS. This suggests the participation of a Cl-/HCO3- exchange in the recovery from alkalosis in rat myocardial cells that is not changed by diabetes. On the other hand, the amplitude of the acidification induced by the withdrawal of NH4+ was markedly increased in diabetic papillary muscles as compared to normal muscles. Moreover, there was a marked slowing down of the recovery from acidosis in the diabetics. The amplitude of NH+4 withdrawal-induced acidification was increased equally by amiloride in both normal and diabetic muscles. These findings suggest that diabetes is associated with a change in the activity of the amiloride-sensitive Na+/H+ exchange.