Thioacetamide (0.01-1.3 mM) fails to exert any significant immediate effect upon insulin release from rat isolated islets. However, when administered (4 micromol/g body wt) intraperitoneally 24 h before sacrifice, it reduced food intake and body weight and affected the secretory response of isolated islets to several secretagogues, despite unaltered insulin content of such islets. This coincided with a decrease in D-[U-14C]glucose oxidation, total islet calcium content and the ionized calcium content of secretory granules in islet B-cells, and changes in both 133Ba and 45Ca net uptake. Likewise, in islets prepared from thioacetamide-injected rats and prelabeled with 45Ca before perifusion, the cationic and insulin secretory responses to D-glucose or gliclazide, but not to the association of Ba2+ and theophylline in the absence of extracellular Ca2+, often differed from that otherwise found in islets prepared from control rats. These findings are interpreted as indicative of an impaired capacity of Ca2+ sequestration by intracellular organelles in the islet B-cells of thioacetamide-treated rats.