The molecular characteristics of thiamin (T) transport were studied in the small intestinal and renal brush border membrane vesicles of rats, using [(3)H]T at high specific activity. The effects of various chemical modifiers (amino acid blockers) on T uptake were examined and their specificity assessed. Treatment with the carboxylic specific blockers 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-p-toluene sulfonate, (1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide hydrochloride and N-ethyl-5-phenylisoaxolium-3'-sulfonate (Woodward's Reagent K) and with the sulfhydryl specific blocker p-chloromercuribenzene sulfonate inhibited T transport in both types of vesicles. Phenylglyoxal, but not ninhydrin, both reagents for arginine residues, and diethylpyrocarbonate, a reagent for histidine residues, specifically decreased T transport only in renal and small intestinal vesicles respectively. Similarly 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole reacted, but not N-acetylimidazole, both of which are reagents for tyrosine residues. However, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole inhibition was aspecific. Acetylsalicylic acid, a reagent for lysine and serine residues, decreased T transport, but the lysine effect was aspecific. Acetylsalicylic acid serine blockage also eliminated T/H(+) exchange in small intestinal vesicles. Taken together, these results suggest that for T transport carboxylic and sulfhydryl groups and serine residues are essential in both renal and small intestinal brush border membrane vesicles. In addition, arginine and histidine residues are also essential respectively for renal and small intestinal transporters. Serine was essential for the T/H(+) antiport mechanism.