The $trans{ ightarrow}cis$ photoisomerization and fluorescence of trans-1,2-bispyrazylethylene were investigated in various conditions. The quantum yields of the $trans{ ightarrow}cis$ photoisomerization and the fluorescence intensity of trans-1,2-bispyrazylethylene decrease on going from neutral to acidic or basic solutions. The quantum yields of photoisomerization, however, are little affected by changing the concentration of salt while the fluorescence intensity increases as the concentration of salt increases. pH and salt effects on the energy levels of $^1(n,;{pi}^*);and;^1({pi},; {pi}^*)$ states lead to opposing changes in photoisomerization and fluorescence quantum yields. The heavy atom effect on the fluorescence of 1,2-bispyrazylethylene was also investigated.