The effects of extracellular divalent cations on the intracellular Ca 2+ concentration ([Ca 2+] i) in neonatal rat islet cells were investigated to determine whether these cells, like several others, have signal-generating surface cation sensors. Raising the external Ca 2+ concentration by 1 mM increments triggered either sustained increases in [Ca 2+] i or large sharp [Ca 2+] i spikes followed by return to a suprabasal level. The external Ca 2+-triggered [Ca 2+] i responses were abolished by treating the cells with the inhibitor of inositol phospholipid hydrolysis, neomycin (1.5 mM), but not by another phospholipase C inhibitor, U-73,122 (2.5 μM), or the voltage-sensitive Ca 2+ channel blockers nifedipine (20 μM) and methoxyverapamil (D600; 50 μM). [Ca 2+] i responses were also triggered by barium (Ba 2+; 1 mM) and cobalt (Co 2+; 1 mM). The Ba 2+ responses were also inhibited by neomycin and unaffected by nifedipine or D600 and the Co 2+ response required external Ca 2+. Therefore, neonatal rat pancreatic islet cells may display divalent cation receptors/sensors on their surfaces. Activation of these putative receptors, which are coupled to neomycin-sensitive, voltage-independent, dihydropyridine-insensitive channels, by Ca 2+, Ba 2+ or Co 2+ would trigger [Ca 2+] i responses by opening these channels to admit external Ca 2+ into the cell. The physiological function(s) of such cell-surface divalent cation receptors/sensors and the [Ca 2+] i surges they generate in pancreatic islet cells is not known.