The properties of the tyrosine and tyrosinate emissions from brain S-100b have been studied by nanosecond time-resolved fluorescence at emission wavelengths in the range 305 to 365 nm. The effect of pH on the fluorescence has been studied at pH 6.5, 7.5, and 8.5 for the Ca(II) apo and holo forms of the protein, and for the apo and holo forms in the presence and absence of Zn(II) at pH 7.5. The fluorescence decay is biexponential at pH 8.5 and triexponential at pH 6.5 and 7.5. The three components of the decay have wavelength and metal ion dependent lifetimes in the ranges 0.06 to 1.05 ns, 0.49 to 3.76 ns, and 3.60 to 14.5 ns. The observation of a long lifetime component at wavelengths characteristic of emission from tyrosinate suggests that in class A proteins this may be a useful diagnostic of the environment of tyrosine in their native structures. The time-resolved emission spectra provide evidence for efficient, subnanosecond protolysis of the excited state of the single tyrosine (Tyr17) under all conditions studied except in 6 M guanidium chloride in which the protein shows only emission from tyrosine (lambda em 305 nm), suggesting that the tyrosinate emission is a property of the tertiary structure of the native protein. The Zn(II)-dependence of the fluorescence is fully consistent with the earlier suggestion that Tyr17 is near the Zn(II) binding site and remote from the high affinity Ca(II) binding site.