The fluorescence emission of the single tryptophan in bovine brain S-lOOa protein has been studied by using time-resolved laser fluorescence spectroscopy. The tryptophan fluorescence emission was isolated by a Schott 0-54 cut-off filter at right angles to the excitation direction by a Hamamatsu R955P photomultiplier. With excitation at 295 nm, the fluorescence decay of S-lOOa protein in 25 mM Tris buffer was best represented by a sum of three exponential terms regardless of solvent conditions. At 20°C and pH 7.2, the three components of lifetime for apo-S-lOOa protein were (tau)1~0.43 ns, (tau)2~1.24 ns, and (tau)3~4.05 ns. The corresponding fluorescence contributions of each component were 31%, 31% and 38%, respectively. When the protein was saturated with Mg2, the lifetimes increased slightly and the contribution of the shortest fluorescence component ((tau)1) to the total emission intensity increased slightly at the expense of the other two components. Binding of Ca2+ to S-lOOa protein resulted in a significant decrease of (tau)1, and a substantial increase of (tau)2 and (tau)3, and an increase of the average of the three lifetimes. Under this condition the fractional fluorescence contributions associated with (tau)2 and (tau)3 increased very significantly at the expense of the shortest component. The fluorescence decay behavior of each of he S-lOOa samples was relatively insensitive to the variation of temperature (4~20°C). At pH 8.2 and pH 8.4, the decay behavior of the protein in response to binding of Ca2+ and Mg2+, and changes of temperature was very similar to those observed at pH 7.2. The triple exponential decay kinetics of the single tryptophan in S-lOOa protein could be rationalized by the existence of multiple local conformers.