Optical absorbance spectra were recorded simultaneously and continuously in the isolated rat pancreas arterially perfused at various flow rates. This was done to explain how optical absorbance changes corresponding to parallel reduction of cytochromes aa3, b, and cc1 were observed in perfused pancreas stimulated with high concentration of an exocrine secretagogue such as cholecystokinin-(26-33) (CCK-8). With perfusion flow rate between 1.5 and 3.0 ml/min, there were no optical absorbance changes corresponding to cytochrome reduction, but these optical absorbance changes occurred when the perfusion flow rate was decreased to 1.0 ml/min. These optical absorbance changes were not observed during exocrine secretion stimulated by CCK-8 (30 pM, 100 pM, and 1 nM) at the perfusion flow rate of 3.0 ml/min. Transient but slight changes in optical absorbances, which correspond to reduction of cytochromes, were observed in the glands perfused at the flow rate of 2.0 ml/min when secretion was stimulated by 1 nM CCK-8. When the perfusion flow rate was decreased to 1.0-1.5 ml/min, these optical absorbance changes corresponding to reduction of cytochromes occurred in glands stimulated by CCK-8 (30 pM, 100 pM, and 1 nM). Optical absorbance changes corresponding to reduction of mitochondrial cytochromes during secretion stimulated with CCK-8 may indicate local hypoxia in the perfused organ.