The literature concerning the extraction and quantification of juglone (5-hydroxy-1,4-naphthoquinone) frequently fails to differentiate between juglone and its immediate precursors. The proposal that the term juglone potential be used to define the total concentration of juglone under an oxidative environment in walnut tissues is made. The juglone potential of a given tissue will more accurately reflect the allelopathic agent''s presence than will juglone concentration of the tissue alone. The term juglone potential accepts the multiple chemical nature of juglone found in walnut tissue and gives a precise way for quantification of allelopathic action. The objective of this study was to measure seasonal changes in black walnut juglone potentials at various locations in the crown. Juglone potentials were measured weekly in leaves at the top, middle, and bottom of the tree crowns. The results showed a linear decrease in juglone potential over the growing season. The results also showed no significant difference in leaf juglone potentials among the three crown positions. If reanalyzed without the middle crown position juglone potentials were significantly greater in the lower crown positions when compared with the higher crown position. Potential use of juglone to manipulate species composition and decrease interference in plant communities is discussed.