Thiamine, in very small concns. (1:10,000), inhibits the vasoconstrictor action of nicotine in frog vessels. The inhibiting action of thiamine is linked with the thiazole moiety of the molecule, the pyrimidine group being ineffective. Thiouracil, nicotinamide and para-amino-benzoic acid do not inhibit the action of nicotine; sulfathia-zole has an inhibiting effect only in a relatively high concn. (4:1000). The site of action of thiamine seems to be at the myoneural junction in the striated muscles and at the post-ganglionic nerve endings or in the muscle elements in the vessel walls in smooth muscles.