Rabbit skeletal alphaalphatropomyosin was specificially labeled at cysteine 190 with the fluorescent reagent, N-(1-pyrene)maleimide. Spectroscopically different products were obtained by labeling at pH 6.0 (PyrI-alphaalphaTm) or pH 7.5 (PyrII-alphaalphaTm). PyrII-alphaalphaTm results from a secondary reaction between the N-(1-pyrene)succinimido moiety at cysteine 190 of PyrI-alphaalphaTm and a lysine group on the same chain, probably lysine 189. Pyrene excimer fluorescence was present in the native state but absent in the unfolded state of both products, thus verifying the proximity of the--SH groups and the chain register model for the structure of tropomyosin. Studies of the guanidinium chloride-dependent unfolding of PyrII-alphaalphaTm showed that loss of excimer fluorescence precedes unfolding, providing evidence for a region of preferential instability in the molecule near cysteine 190. This work suggests that N-(1-pyrene)maleimide could be used to probe both--SH proximity and local conformation in any protein if the presence of two or more proximal--SH groups is suspected.