1. Diabetic modifications of nicotinic receptor-operated noncontractile Ca2- mobilization observed in the presence of anticholinesterase were investigated by measuring Ca(2+)-aequorin luminescence in diaphragm muscles of mice with diabetes induced by injections of streptozotocin (150 mg kg-1, bolus i.v.) and alloxan (85 mg kg-1, bolus i.v.). 2. The diabetic state accelerated the decline of noncontractile Ca2+ transients without affecting their peak amplitude. Insulin treatment reversed this alteration. 3. The increase in contractile Ca2+ transients by cholinesterase inhibition was attenuated 0.6 fold and became resistant to changes in [Ca2+]o in the diabetic state. 4. Changes in extracellular pH from 7.6 to 5.6 depressed the peak amplitude of noncontractile Ca2+ transients without affecting their duration, and enhanced the peak amplitude of contractile Ca2+ transients. 5. These results suggest that the inactivation process of noncontractile Ca2+ mobilization is promoted in diabetic muscles, presumably by desensitization of the nicotinic acetylcholine receptor.