In dimethyl sulfoxide, the aminolysis of p-nitrophenyl acetate by aminopyridines results in amide formation, through nucleophilic catalysis by the ring nitrogen for 4-aminopyridine, but by direct amino nitrogen attack for 2-aminopyridine (as previously found for acetic anhydride). In water, the aminopyridines catalyse the hydrolysis of the ester (unlike aniline, which still gives acetanilide). In general, this occurs by nucleophilic catalysis by the ring nitrogen. Even 4-amino-2- methylpyridine reacts by this route (though 2-picoline does not) and, of the compounds studied, only for 2-amino- 6-methylpyridine does general base catalysis occur instead. Reasons for these mechanism changes are discussed.