In the polyclonal rat pancreatic beta -cell line INS-1, immunoreactive insulin (IRI, insulin and its precursors) and C-peptide (surrogate marker for mature insulin) were quantified after a 1-h incubation at 16.7 mM glucose with or without oleate. Oleate caused a 20% decrease (P less than or equal to 0.01) and a shift towards mature insulin (P less than or equal to 0.01) in intracellular IRI. As IRI secretion was significantly (P less than or equal to 0.01) diminished in the presence of oleate at hyperglycemic conditions the loss of intracellular IRI had to be due to other reasons which was then examined by focussing on newly synthesized, radiolabelled IRI*. Thereby, oleate caused a 50% decrease in newly synthesized intracellular IRI* (P less than or equal to 0.01) and-contrary to the findings in complete intracellular IRI-a shift towards immature insulin precursors. Taken together, exposure of INS-1 cells to oleate induces an intracellular lack of uncompletely processed IRI. This is due to an inhibited biosynthetic supply which can not compensate for intracellular IRI-degradation and (to a neglible extent) for insulin secretion.