The development of gap junctions between insulin-containing B cells was quantitatively analyzed in islets of Langerhans isolated from rats treated with the sulfonylurea glibenclamid for 1, 2, or 7 days. Glibenclamid treatment was associated with a marked depletion of the insulin content of B cells and with an increase in the number and size of gap junctions between these cells. A significance correlation was found between these two events.