Islets isolated fromob/obmice which had been fed a vitamin D‐deficient diet released significantly less insulin in response to glucose than did vitamin D‐replete islets but showed normal net45Ca2+uptake. To determine whether vitamin D3has a direct effect on the pancreatic B cell, islets fromob/obmice on a normal diet were exposed to vitamin D,in vitrofor 1 week or only 3 h, and then glucose‐stimulated45Ca2+uptake and insulin release were measured. Exposure to 1 nM or 1 μM vitamin D3for 1 week stimulated45Ca2+uptake in the presence of 3 mti, but not 20 mM glucose, and did not affect insulin release. Exposure to vitamin D3for 3 h did not significantly increase net45Ca2+uptake although there was a tendency to such an effect (P= 0.10). In conclusion, vitamin D‐deficiencyin vivosuppressed subsequent glucose‐stimulated insulin releasein vitroand this effect may be due to a direct effect of the sterol (or one of its metabolites) on calcium handling by