The insulinic response of rats challenged with glucose at different times after an oral dose of 40 µmol NaF/100 g bw revealed the concentration of the molecular species reacting with anti-insulin antibody was significantly lower after 3 hr, and the glucose levels were somewhat higher than in controls. At the 4th and 5th hr, plasma F levels were still higher than in the controls, at which times the response of B-cells toward glucose was assessed by a dose-response function. The concentration of glucose that produced 50% of the insulinic response was significantly higher in rats treated with NaF (6.3±0.2 g/L) compared with the control group (4.3±0.4 g/L), P<0.05. These results indicate that F-treated rats exhibit a significantly reduced sensitivity to glucose stimulus. Measurements of plasma insulin at the 5th hr after F load by gel filtration of plasma, followed by measurement of insulin in the fractions containing 5–8 kDa peptides, revealed that only one-fifth of the species reacting against the antiserum corresponds to this hormone. The secretion of C-peptide did not exhibit the expected relationship with immunoreactive insulin-like species. The clearance of intravenously injected 125I-insulin was not affected by the presence of fluoride. The incorporation of 3H-leucine into B-cell proteins in the range of 5 to 13 kDa was higher in F-treated than in control slices, supporting the hypothesis that F does not interfere with the synthesis of insulin. This work provides further data on the dysfunction induced on pancreatic B-cell physiology by increased F in the extracellular fluid, a dysfunction that most probably involves disturbance of the enzymatic processing of insulin precursors.