Isatin is an endogenous indole present in mammalian tissues and fluids. This study was designed to test the effect of both acute stress (AS) and gender on isatin's levels in rat serum, heart and brain, using a HPLC-UV detection method. Basal brain isatin levels were higher in females than in males. AS resulted in increased isatin levels in male serum, heart and brain, and in female serum and heart, but not in the brain. The percentage increases were greater in males than in females in all three sources. After stress, there were significant correlations between isatin levels in the serum and the heart. However, there were no correlations between brain levels and those in the other sources. These results consolidate previous evidence that isatin levels are altered by stress and suggest that brain isatin is under separate control from that in the periphery.