Uptake of 45Ca2+ by a microsomal fraction from isolated pancreatic islets of non-inbred ob/ob mice was studied. ATP strongly stimulated 45Ca2+ uptake, the maximum effect being obtained with 2mM-ATP. GTP and CTP at this concentration did not increase the uptake. Scatchard analysis revealed at least two types of uptake mechanisms in the presence of 2 mM-ATP; the apparent association constants were 1.1 x 105rvm1 and less than 2.5 x 102M-1. In contradistinction to an unaffected low-affinity uptake, the highaffinity uptake was drastically decreased on omission of ATP. The ATP-dependent and high-affinity uptake was half-saturated at about 10-20M-Ca2+ and was inhibited by 10 or 100puM of cyclic AMP, 10pum cyclic GMP, 10puM cyclic CMP, or 5mM-theophylline.